query method in an api.py also needs retry in some cases (for example
when we are running is_healthiest_node check).
In all cases we should retry only when connection is closed or broken.
BUT, the connection status must be checked via cursor.connection (old
implementation was using general connection object for that). For
multi-threaded applications this is not appropriate, because some other
thread might restore connection.
In addition to that I've changed most of the unit tests to use `Mock` and
`patch` where it is possible.
In case if one member of a cluster is not available it will retry with
another one and fetch the new cluster configuration. Default timeout for
all requests to etcd is 5 seconds.
Initial cluster configuration can be resolved through:
1) /v2/members call on one of the cluster members on a client port
2) when it is possible to resolve hostname into multiple ip's it will
iterate through list and try to perform action from 1)
3) If there is discovery_srv defined in etcd section of config file it
will resolve peer addresses of all cluster members and will fetch
cluster configuration with using peer protocol by doing /members call on
a peer port
Also check presence of trigger_file on master after promotion when
pg_is_in_recovery() = false and if it is there - remove it.
Plus check presence of trigger_file on a new slave (after running
pg_basebackup) and if it is there - also remove it.
Alexander Kukushkin