odb: split `struct odb_transaction` into separate header

odb: split `struct odb_transaction` into separate header

The current ODB transaction interface is colocated with other ODB
interfaces in "odb.{c,h}". Subsequent commits will expand `struct
odb_transaction` to support write operations on the transaction
directly. To keep things organized and prevent "odb.{c,h}" from becoming
more unwieldy, split out `struct odb_transaction` into a separate
header.

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