ofproto: Drop 'expirable_mutex' in favor of new global 'ofproto_mutex'.

This is the first step toward making a global lock that protects everything
needed for updating the flow table.  This commit starts out by merging one
lock into the new one, and later commits will continue that process.

The mutex is initially a recursive one, because I wasn't sure that there
were no nested acquisitions at this stage, but a later commit will change
it to a regular error-checking mutex once it's settled down a bit.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>