Enable batch options to be set in config for internal sync

Allows an operator to set batch_size and batch_count for internal push
replications to a specific node via config:set, for example:

config:set("mem3.batch_size", "dbcore@db1.cluster001.cloudant.net", "200").
config:set("mem3.batch_count", "dbcore@db1.cluster001.cloudant.net", "2").

This may prove useful when syncing a new node into the cluster.