REST listener improvements for liteserv

[snej]

This branch contains all the LiteCore enhancements & fixes needed to implement the liteserv server.

• Implemented the _changes endpoint of the REST API, all 3 modes (normal, longpoll, continuous.) The latter two modes often have to block waiting for new changes; the implementation using Listener::Task ensures these idle connections aren't blocking a thread or locking a database, so it should be very scalable.
• Implemented a new _query endpoint for running queries.
• Lots of changes to the _replicate endpoint:
  • collection support
  • channel & docID filter support
  • doesn't block until the replication completes (that way the client gets the session_id and can cancel it)
  • simplified the cancel API to take the session_id
  • added a C4Listener method to start a replication task
• I made a bunch of improvements to the RequestResponse class, like chunked-encoding support, and the ability to flush output to the socket before finishing the request (so long responses don't allocate a lot of memory.)

[cbl-bot]

Code Coverage Results:

Type	Percentage
branches	66.42
functions	79.0
instantiations	33.64
lines	77.93
regions	73.76