This commit adds a new SendState struct, which is used internally by the
txmgr to determine if we should give up on a particular tx at the given
nonce due to ErrNonceTooLow failures. To do so, we track the set of
mined txns at any particular time, which are updated via asynchronous
calls to TxMined and TxNotMined by each of the spawned goroutines. The
presence of a mined txn supercedes any of the errors returned, and will
cause the txmgr to wait for full confirmation. However, if we observe
multiple ErrNonceTooLows, the SendState will wait for a configurable
safe abort count to ensure that the reading wasn't due to being in a
transient state, as we don't have exact guarantees on the execution
ordering of various goroutines.

With this change, the test added in the prior commit now succeeds.