ovn-controller: Add a new thread in pinctrl module to handle packet-ins.

Prior to this patch, ovn-controller was single threaded and everytime the
poll_block() at the end of the main while() loop wakes up, it  processes
the whole SB DB and translates the logical flows to OF flows.

There are few issues with this -

  * For every packet-in received, ovn-controller does this translation
    resulting in unnecessary CPU cycles.

  * If the translation takes a lot of time, then the packet-in handling
    would get delayed. The delay in responses to DHCP requests could
    result in resending of these requests.

This patch addresses these issues by creating a new pthread in pinctrl module
to handle packet-ins. This thread doesn't access the Southbound DB IDL object.

Since some of the OVN actions - like dns_lookup, arp, put_arp, put_nd
require access to the Southbound DB contents and gARPs, periodic IPv6 RA
generation also requires the DB access, pinctrl_run() called by the main
ovn-controller thread accesses the Southbound DB IDL and builds the local
datastructures. pinctrl_handler thread accesses these data structures
in handling such requests. An ovs_mutex is used between the pinctr_run() and
the pinctrl_handler thread to protect these data structures.

Acked-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Numan Siddique <nusiddiq@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>