Create a `last_fullsync_completed` statistic

[angrycub]

A customer has requested that there be a last_fullsync_completed statistic because the last_fullsync_started statistic clears itself out when there is not a currently running fullsync.

[angrycub]

See issue at https://basho.zendesk.com/agent/tickets/8987

[engelsanchez]

Micah has worked on a solution for this in #642 that we'll review and try to get in 2.0.4