- Use dbsyncer (SQLCipherU1DBSync) instead of SQLCipherDatabase
  as only the first one supports multiple threads while syncing
  and is actually used by Soledad.sync

database_security parameter was either undocumented or incomplete. This
commit adds a few more doc to make it consistent with latest changes.
Closes #7689

All batching code has no effect by default with this commit. Since we
know that this is a dangerous new feature we will enable them only on
our test servers and check them manually before setting it as default
or adding more configuration features.
Use SyncTarget and server conf file to enable it for testing.

Generation cache was removed for simple processing and it should not got
back, but during a batch the server wont change its generation. So a
little trick to hold that temporary information until batch finishes is
needed.

Batch support is optional. This commit adds a 'batching' configuration
option to disable it.