Implement the "sanity checks" described in b/9509040: Incoming packets

Implement the "sanity checks" described in b/9509040:  Incoming packets
whose connection ID is not associated with a session and is not recorded
as being in time-wait state are tested for reasonableness before a
session is created for them.  This won't protect much against deliberate
DoS but will screen out a reasonable fraction of truly junk packets.

Add sanity tests so we do not create connections for initial packets
with truncated connection IDs.

Merge internal change: 87831756

R=rch@chromium.org

[Ryan Hamilton, 2015-03-07 03:17:57]

lgtm