Expose FLUSHALL to the python client

[ctandon11]

Description

Give the python client access to the Redis command FLUSHALL by exposing it
through the Pybindings.

Justification

Users will be able delete all the keys of all the existing databases from the
python client.

Implementation Strategy

The C++ client has this functionality so just needs to be exposed through
the Pybindings and pyclient.cpp file to the python client layer. This will
be an 'AddressAllCommand' that function repeatedly calls on FLUSHDB
with every node in a cluster. We will need to change AddressAnyCommand
to AddressAllCommand which instead of running commands on one active
node in the cluster, runs on every node in the cluster.