Txmgr always returns first tx even if resubmitted

[corver]

Problem to solve:

• lib/txmgr.*simple.doSend always returns the first tx crafted from the candidate.
• If the tx is re-crafted, and the resubmitted tx gets mined, it returns the incorrect tx.
• This can and did result in incorrect tx-not-mined errors.

Proposed solution:

• Return the actual mined tx