Pull initial chain compaction out of init() and into the syncer

[antiochp]

We added a chain.compact() on startup in #2127 to ensure nodes that did not run 24/7 still had the opportunity to compact their local storage (like a laptop that is running only intermittently).

This PR pulls this initial chain.compact() call out from Chain::init() and into the syncer, specifically on initial transition to NoSync.
It never really belonged there as Chain::init() should not be responsible for going off and cleaning local storage up as it starts up.

This postpones the compaction process to after the initial sync, rather than having it run before we start syncing.

Additionally we add a 60 block threshold to prevent a node triggering multiple chain compactions if restarted frequently (like we do during dev and testing).

This should make the node a bit more responsive initially and should allow a node to begin syncing a little quicker on startup.

Example where we skip chain compaction because we recently ran one -

20190409 16:18:48.975 DEBUG grin_chain::chain - compact: head: 120396, tail: 110315, diff: 10081, horizon: 10080
20190409 16:18:48.975 DEBUG grin_chain::chain - compact: skipping compaction - threshold is 60 blocks beyond horizon.

[ignopeverell]

LGTM!