odb/source-inmemory: implement `free()` callback

odb/source-inmemory: implement `free()` callback

Implement the `free()` callback function for the "in-memory" source.

Note that this requires us to define `struct cached_object_entry` in
"odb/source-inmemory.h", as it is accessed in both "odb.c" and
"odb/source-inmemory.c" now. This will be fixed in subsequent commits
though.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>