Zipdownload: various improvements to this plugin

[gurnec]

This includes several improvements to zipdownload. Feel free to cherry-pick what looks useful, or ask me to split this into multiple PRs, or whatever makes most sense.

• Currently the entire zip is written to the output buffer before sending which can easily cause an OOM. Instead, flush every 512 KiB.
• Update the .htaccess, see 1972a9e for details.
• For maildir exports, change the modified-time of each .eml file to the IMAP internal time to allow sorting extracted emails.
• Change the default charset in the config to UTF-8, see 286c893.
• The 1b55aba refactor commit shouldn't change any behavior (if it does it's a bug), it exists solely to make the later commits easier to follow.

The biggest change is adding optional support for the ZipStream library. This allows streaming emails from the storage backend to the web server while zipping them without any temporary files and without much additional RAM overhead.

It's feasible to download entire INBOXes of multiple GB with this update. Without, temporary disk space of around 130% of the total email size is required.

Potential issues so far:

• No config option to choose which method, ZipArchive or ZipStream. Instead ZipStream is always chosen if available with a fallback to ZipArchive which may be good enough?
• ZipArchive is required to show the menu items even when ZipStream is available. This is done to avoid autoloading ZipStream just for displaying the menu, though it's not ideal.
• No new unit tests, though E2E does test basic ZipStream downloads which may be sufficient?
• No documentation yet, should probably add something to the zipdownload config or README. And CHANGELOG.md?
• I added maennchen/zipstream-php as a suggest to the main composer.json, I'm not sure if I should have only added it to the zipdownload composer.json?
• Is there any interest in adding maennchen/zipstream-php to the Makefile for the complete .tar?

I'm happy to make any requested changes given some feedback. In the mean time, I'll loosely quantify the "without much additional RAM" claim in a follow-up.

[gurnec]

This is the result of entirely unscientific testing on a small VM, running Dovecot/Apache/php-fpm and little else, with one vcpu and about 600MB free RAM. I ran each test a few times in a row, which may have warmed up the disk cache, and took the best results from each.

I recorded three metrics for each test (the column groups): RAM used in MB, time taken in seconds, and zip files size in MB. For time and RAM, I took them from the php-fpm status page.

I ran three test scenarios (the columns):

1. Attachment download - an email with two attachments, one 24.5 MB and one 0.5 MB (the worst case scenario given a max message size of about 34 MB)
2. Maildir export, all emails
3. Mbox export, all emails

I tested four implementations (the rows), each also included PR #10151.

1. ZipArchive - nothing else
2. Zipstream w/strings - the "initial ZipStream support" commit ab7d10a
3. Zipstream w/memory - a commit (not online) which uses php://memory files instead of strings
4. Zipstream w/fibers - this entire PR

The email corpus (for test scenarios 2 & 3) includes:

• 10996 emails
• 830 MB
• 389 "From " lines which needed escaping
• 4 emails > 34 MB in size (these are the worst-case emails which push up memory usage for implementations 2 & 3)
• 67 emails > 512 KB in size

I would expect implementations 2 & 3 to scale upwards in RAM usage in proportion to the size of the largest email (or attachment for that test) included in the download, and for implementations 1 & 4 to remain constant.

Finally I should note that it's "unfair" to compare ZipArchive to ZipStream given that the former uses its default zip compression level (of 9), and the latter specifies a level of 1.

Here are the results:

	Attach. RAM	Maildir RAM	Mbox RAM	Attach. time	Maildir time	Mbox time	Attach. zip	Maildir zip	Mbox zip
ZipArchive	34	10	16	1.5	89.6	84.5	25	331	324
Zipstream w/strings	119	131	n/a	2.2	53.2	n/a	25	354	n/a
Zipstream w/memory	95	97	95	2.6	54.0	56.6	25	354	348
Zipstream w/fibers	26	28	34	1.8	51.2	48.6	25	354	348