rpctest: Correct several issues in tests and joins.
rpctest: Correct several issues in tests and joins.

Contains the following upstream commits:
- c3d5371615c4869e3f899a355421089318976944
  - Reverted since Travis is already at a more recent version
- 7c4b169faa134b6716f574cff87d2e4340fba57b
  - Reverted since this is already done via the Decred docker setup
- 47ced81d440595b0bbe6c355236ac306e107c259
  - Reverted since it has already been fixed by connmanager sync
- 6cf60b5fae5cd076c863892038840a2327e345ce

Also, the merge commit contains the necessary Decred-specific alterations.

jrick/dcrd,jrick/btcd,jrick/dcrd,jrick/btcd,jcmincke/dcrd,decred/dcrd,decred/dcrd,jcmincke/dcrd,jrick/btcd,jcmincke/dcrd