object-file: relocate ODB transaction code

object-file: relocate ODB transaction code

The bulk-checkin subsystem provides various functions to manage ODB
transactions. Apart from {begin,end}_odb_transaction(), these functions
are only used by the object-file subsystem to manage aspects of a
transaction implementation specific to the files object source.

Relocate all the transaction code in bulk-checkin to object-file. This
simplifies the exposed transaction interface by reducing it to only
{begin,end}_odb_transaction(). Function and type names are adjusted in
the subsequent commit to better fit the new location.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>