bulk-checkin: rebrand plug/unplug APIs as 'odb transactions'

bulk-checkin: rebrand plug/unplug APIs as 'odb transactions'

Make it clearer in the naming and documentation of the plug_bulk_checkin
and unplug_bulk_checkin APIs that they can be thought of as
a "transaction" to optimize operations on the object database. These
transactions may be nested so that subsystems like the cache-tree
writing code can optimize their operations without caring whether the
top-level code has a transaction active.

Add a flush_odb_transaction API that will be used in update-index to
make objects visible even if a transaction is active. The flush call may
also be useful in future cases if we hold a transaction active around
calling hooks.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>