Add specific config knob for the size of AppendEntries

Until now the knob shard-snapshot-chunk-size was used to determine
the max size of snapshot chunks as well as AppendEntries messages.
These objects however play roles in two different processes:

• snapshot capture and installation
• journal replication

Adding a separate config knob for the max size of AppendEntries messages will allow the user to tweak the two systems independently and more precisely.