Merge #15773: test: Add BitcoinTestFramework::sync_* methods

fafe5f0d09 test: Remove unused imports (MarcoFalke)
fa16a09215 scripted-diff: use self.sync_* methods (MarcoFalke)
faf77f9b90 test: Pass self to test_simple_bumpfee_succeeds (MarcoFalke)
fa6dc7c5c3 test: Add BitcoinTestFramework::sync_* methods (MarcoFalke)
fafe008cb4 test: Pass at most one node group to sync_all (MarcoFalke)
fa4680ed09 scripted-diff: Rename sync_blocks to send_blocks to avoid name collisions and confusion (MarcoFalke)

Pull request description:

  This adds methods to the test framework that can be called by just `self.sync_*()`.

  This avoids having to import the underlying util method. Also, in the default case, where all nodes are synced this avoid having to pass `self.nodes` explicitly.

  So the effective changes are:

  ```diff
  @@
  -from test_framework.util import sync_blocks, sync_mempools
  @@
  -        sync_blocks(self.nodes)
  +        self.sync_blocks()
  @@
  -        sync_mempools(self.nodes)
  +        self.sync_mempools()

ACKs for commit fafe5f:
  promag:
    utACK fafe5f0.
  jonatack:
    ACK https://github.com/bitcoin/bitcoin/pull/15773/commits/fafe5f0d09db6778173a1079d93c314e9317708c, nice simplification.

Tree-SHA512: 5c81840edf9fb3c5de2d7bf95ca36a5a8d23567cd1479a0f4044547c2080e9a3c5cf375357bc8eebb5b68829be050a171ab2512cfd47b89feed51fe3bad2cd72

test/functional/wallet_resendwallettransactions.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-# Copyright (c) 2017-2018 The Bitcoin Core developers
+# Copyright (c) 2017-2019 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test that the wallet resends transactions periodically."""