In (strCommand == "tx"), return if AlreadyHave()
In (strCommand == "tx"), return if AlreadyHave()

The main effect is to exit processing for recently-rejected hashes,
in case they are pushed to us without prior advertisement.  This
behavior was seen in the wild.

An additional effect is to do early checks for mempool or mapOrphan
existence.  No logging or nDoS tracking is needed for failures of
these checks.

syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin