These functions will acquire users in future commits.

This new functionality made the internal state machine of the log hard
enough to understand that I thought that it was best to make it explicit
with a 'state' variable, so this commit introduces one.

This commit duplicates code and logic from ovsdb_rename() and
ovsdb_compact() in ovsdb/file.c.  A future commit removes this duplication.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>