- Related: #6359

Instead of opening one TCP connection for each HTTP request, we want to reuse
connections. Also, we need to be able to verify SSL certificates. This commit
implements both features in the twisted http client sync.

This change uses twisted deferreds for the whole syncing process and paves the
way to implementing other transport schemes. It removes a lot of threaded code
that used locks and was very difficult to maintain, and lets twisted to the
dirty work. Furthermore, all blocking network i/o is now handled
asynchronously by the twisted.

This commit removes the possibility of interrupting a sync, and we should
reimplement it using cancellable deferreds if we need it.

This commit actually does some different things:

  * When doing asynchronous decryption of incoming documents in soledad client
    during a sync, there was the possibility that a document corresponding to
    a newer generation would be decrypted and inserted in the local database
    before a document corresponding to an older generation. When this
    happened, the metadata about the target database (i.e. its locally-known
    generation) would be first updated to the newer generation, and then an
    attempt to insert a document corresponding to an older generation would
    cause the infamous InvalidGeneration error.

    To fix that we use the sync-index information that is contained in the
    sync stream to correctly find the insertable docs to be inserted in the
    local database, thus avoiding the problem described above.

  * Refactor the sync encrypt/decrypt pool to its own file.

  * Fix the use of twisted adbapi with multiprocessing.

Closes: #6757.

Since we started implementing twisted api in soledad, some pieces are missing.
Accessing the sqlcipher database directly with the twisted adbapi facilities
is one of them. The async encryption/decryption was touching the database
directly, and this was causing some difficulties like having different threads
accessing the same database. This commit implements the twisted adbapi stuff
for the asynchronous encryption/decryption facilities.

Next steps would be use async adbapi for async encryption and use async adbapi
for all sqlcipher access.

We always got a log message saying "canceling sync threads" in the end of the
sync process, even when there was no error during the sync. This commit
changes that in a way that we only have that log when the sync was actually
cancelled because of an error.

Both deferred encryption and decryption rely on a special sync db. Previous to
this fix, the sync db was only initialized if a syncer was configured with
deferred encryption capabilities. This was a problem when the syncer was not
configured like so, but the actual sync method was initiated configured to do
deferred decryption.

This commit fixes this by always initializing the sync db, so we have the
option of doing all combinations of deferred encryption and decryption.