Properly handle empty before/after notifications in l2pop code

Change-Id: I8644bb7cc2afb3b181397a478f96927990c0a4ca
Closes-Bug: #1367881
ofagent: Fix a possible crash in arp responder

Be careful for exceptions when feeding packet-in data,
which is generated by tenant VMs and thus can not be trusted,
to Ryu packet library.

Closes-Bug: #1365255
Change-Id: Ia8bacfb55def563a1b23a47709ae72bd4fce0fce
Avoid constructing a RouterInfo object to get namespace name

Constructing a RouterInfo object just for a string concatenation is
inefficient and adds more dependence on the class which needs
refactoring.

Change-Id: Ibaf369d6ebe9285a0c845802def59bfa26ac0fd5
Race for l2pop when ports go up/down on same host

With l2pop enabled, race exists in delete_port_postcommit
when both create/update_port and delete_port deal with
different ports on the same host, where such ports are
either the first (or) last on same network for that host.
This race happens outside the DB locking zones in
the respective methods of ML2 plugin.

To fix this, we have moved determination of
fdb_entries back to delete_port_postcommit and removed
delete_port_precommit altogether from l2pop mechanism
driver.  In order to accomodate dvr interfaces, we
are storing and re-using the mechanism-driver context
which hold dvr-port-binding information while
invoking delete_port_postcommit.  We loop through
dvr interface bindings invoking delete_port_postcommit
similar to delete_port_precommit.

Closes-Bug: #1372438
Change-Id: If0502f57382441fdb4510c81a89794f57a38e696
Merge "Race for l2pop when ports go up/down on same host"
Merge "ofagent: Fix a possible crash in arp responder"
Merge "Avoid constructing a RouterInfo object to get namespace name"
Merge "Properly handle empty before/after notifications in l2pop code"