Dispose db connections pool on disconnect

When a disconnect happens (e.g. when VIP moves to another node), all
existing connections in the pool become invalid. There is no sense to
check every single connection in the pool, as this may take a long
time (e.g. when python-mysqldb driver is used with eventlet green
threads, there is no context switch on db IO, so the whole process is
blocked until socket read times out). Dispose all connections in the
pool when disconnect is detected, they will be recreated on demand
then.

Closes-Bug: #1288438

Change-Id: Ia357da2b3092d306a86b6d1787bab374a335f28c
Merge "Dispose db connections pool on disconnect"