Removing "master_nodes" module variable

[mrgonza78]

Removing "master_nodes" module variable b/c:

• info in this variable was not "per client" (like slot_cache) so it was containing a list of all master nodes of all cluster-clients ever instantiated
• list was always growing (there were just appends to the table)
• list of master nodes can be already obtained from the slots info already cached per client (this eliminates the need of an extra "nodes" query to redis)

I'm missing testing for this. I could not find in the readme or anywhere how tests are run for this project. If we can add those instructions I could run existing tests and add new ones that verify what I'm actually fixing.

Thanks