PushInventory() is currently called with a CInv object, which can be a
MSG_TX or MSG_BLOCK. PushInventory() only uses the type to determine
whether to add the hash to setInventoryTxToSend or
Since the caller always knows what type of inventory they're pushing,
the CInv is wastefully constructed and thrown away, and tx/block relay
is being split out, we split the function into PushTxInventory() and
Backport of core PR19322.