Merge branch 'jt/odb-transaction-write' into ps/odb-in-memory

* jt/odb-transaction-write: odb/transaction: make `write_object_stream()` pluggable object-file: generalize packfile writes to use odb_write_stream object-file: avoid fd seekback by checking object size upfront object-file: remove flags from transaction packfile writes odb: update `struct odb_write_stream` read() callback odb/transaction: use pluggable `begin_transaction()` odb: split `struct odb_transaction` into separate header