odb: add transaction interface

Transactions are managed via the {begin,end}_odb_transaction() function
in the object-file subsystem and its implementation is specific to the
files object source. Introduce odb_transaction_{begin,commit}() in the
odb subsystem to provide an eventual object source agnostic means to
manage transactions.

Update call sites to instead manage transactions through the odb
subsystem. Also rename {begin,end}_odb_transaction() functions to
object_file_transaction_{begin,commit}() to clarify the object source it
supports.

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

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