odb/transaction: add transaction interface to write packfiles

In git-receive-pack(1), the incoming packfile is written to the ODB via
`unpack()`, which spawns git-index-pack(1) or git-unpack-objects(1)
directly. With pluggable object databases, an alternative backend may
need to handle writing packfile data differently though.

Introduce `odb_transaction_write_pack()` as a generic interface to
handle writing a packfile to a transaction and use the logic from
`unpack()` as the "files" backend implementation. Note that when storing
the objects as a packfile, git-index-pack(1) also writes a ".keep"
lockfile next to it to prevent a concurrent repack from removing the new
pack prior to reference updates being performed. The "files" transaction
backend is responsible for managing these ".keep" files and removes them
post-commit once the transaction is finalized.

Call sites in git-receive-pack(1) are updated accordingly.

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

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