[tcp] Extraneous Hash Table Lookups from QDesc to Connection State

[BrianZill]

Currently, getting from a Queue Descriptor to an Established connection takes two separate hash table lookups. These lookups occur for several different fast-path API calls, and presumably could be reduced to a single hash table lookup, or something even faster.

[ppenna]

@BrianZill this is related to TCP, the other stacks and LibOSes don't have this.

[iyzhang]

This is no longer the case now that we have unified on NetworkTransport's SocketDescriptor

[ppenna]

@iyzhang, as far as I recall, in most places as Brian suggested, we could invert the casting logic. Could you point us to at least three points in the code where such casting is not possible?