Add response compression to replicator

[lacklacklack]

Overview

Adds gzip compression for outbound HTTP responses when the client sends
Accept-Encoding: gzip.

The replicator now sends Accept-Encoding: gzip on outbound requests
(_bulk_docs, _revs_diff, _bulk_get) and automatically decompresses
gzip responses received from the remote server.

Testing recommendations

Run a replication and check the stats:

# Start CouchDB
./dev/run --admin=admin:admin

# Create DBs, add docs, replicate
curl -X PUT http://admin:admin@127.0.0.1:15984/source_db
curl -X PUT http://admin:admin@127.0.0.1:25984/target_db
curl -X PUT http://admin:admin@127.0.0.1:15984/source_db/doc1 \
  -H "Content-Type: application/json" \
  -d '{"data":"'$(python3 -c "print('x'*2000)")'"}'
curl -X POST http://admin:admin@127.0.0.1:15984/_replicate \
  -H "Content-Type: application/json" \
  -d '{"source":"http://admin:admin@127.0.0.1:15984/source_db","target":"http://admin:admin@127.0.0.1:25984/target_db"}'

# Check stats — value should be > 0
curl -s http://admin:admin@127.0.0.1:15984/_node/_local/_stats/couch_replicator/responses_decompressed | python3 -m json.tool

Automated tests: couch_replicator_compression_tests in src/couch_replicator/test/eunit/.

Related Issues or Pull Requests

Add request compression to replicator - #6013

Checklist

• This is my own work, I did not use AI, LLM's or similar technology
• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• Documentation changes were made in the src/docs folder
• Documentation changes were backported (separated PR) to affected branches

[nickva]

It's a good start! But I think we'll still need the main bits on the server that do the compression. To make things much easier for us mochiweb already includes options to compress responses both for chunks and one-shots.

Chunks may be neat to try but a bit tricky at first. Let's see if we can do one-shot ones at least.

[nickva]

See how we use mochiweb_header make/get_value that has a facility to fetch headers in a case insensitive way

[nickva]

minor nit: _revs_diff and _bulk_docs calls are for the target