Simplify creating queries returning the whole view dataset (empty queries).
Implement QueryView model api automaticly exposing a way to fetch children
using one of its fields as index.
Typo fix.
Disconnect couchdb connection when shutting down agency.

This is done for the sake of leaving reactor clean in tests.
Don't try to reconnect after disconnect() method is called (in couchdb driver).
Fix concurrency issues on txamqp channel when flushing pending messages after
the reconnection.

This problem was caused by the recent fix of the memleak in the same module.