protocol: don't check MaxTime on AddTx
protocol: don't check MaxTime on AddTx

We can't check the transaction's MaxTime in AddTx because:

  a) We're not guaranteed that our clock is in sync with the generator's
     clock.
  b) We're not guaranteed that the tx wasn't already published in a
     block. To ensure idempotence, an already-published tx with an
     elapsed max time should still accepted into the pool.

Closes chain/chainprv#1767

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain