Improvements for system-wide document purge

[adamcfraser]

Currently CBL and SG manage document purge independently:

• TTL set on the document in Couchbase Lite will purge that document
• expiry set on the document in Sync Gateway will purge the document (non-convergence)
• server metadata purge interval will purge the document (convergence)

This can lead to unwanted scenarios when documents are purged on one end of a replication, and that replication is restarted from zero. Should evaluate options to simplify/improve document purge management throughout the system.

[adamcfraser]

Closing - not prioritized.