OFConnectionManager: dont dereference null pointer when logging about a disconnected cxn
OFConnectionManager: cleanup unnecessary uses of connection ID

Every time we do a lookup from connection ID to connection_t *, we need to do a
NULL check. But whenever we do have a connection_t *, we know that it is valid
and active. These changes make the code more foolproof by using the pointer
when we already have it.
OFConnectionManager: handle EAGAIN from socket write

We usually detect socket buffer full from short writes. However, in the case we
exactly filled the socket buffer in the previous write, the next write will
return EAGAIN.

This could also be a "break" instead of checking against the output list, but I
did't think this rare case was worth another control flow path.
OFConnectionManager: assert connection exists when pausing
OFConnectionManager: log connection IDs when lookups fail

Also logs the ID on state changes so we have some context for the failure. The
ID is logged as "index:generation" because it's shorter and easier to read than
the hex version.
Merge into master from pull request #163:
Fix bugs in logging and socket write (https://github.com/floodlight/indigo/pull/163)