Tests for replicator use_checkpoints option
Add documentation for use_checkpoints replicator option
docs: various updates for release notes for 1.6.0

- closes COUCHDB-1795
- closes COUCHDB-1962
Document use_checkpoints and checkpoint_interval config options.
don't crash if ibrowse is already started

fix #COUCHDB-1986
Improve docs of use_checkpoints and checkpoint_interval config options.