Use 'replica' instead of 'slave'

The term 'slave' is very mysql specific. Deprecate the API terminology
that references 'slaves', and 'slave_of', and replace it with 'replicas',
and 'replica_of' instead which is more applicable across datastores of
different types.

Co-Authored-By: Greg Lucas <glucas@tesora.com>
Change-Id: Ief4bc83369fa3183a321d3e07b69b1801595ab15
Closes-bug: 1360310
Merge "Use 'replica' instead of 'slave'"