Make mining fee policy match relay fee policy.

This resolves a case in which a mismatch could be used to bloat up the
mempool by sending transactions that pay enough fee to relay, but not
to be mined, with the default policies.
qt: Make it possible again to specify -testnet in config file

Changes for the datadir chooser have made it impossible to specify
the network (testnet/regtest) in the configuration file for the GUI.

Reorganize the initialization sequence to make this possible again.

- Moves the "datadir" QSetting so that is no longer dependent on the
  network-specific application name (doing otherwise would create a
  chicken-and-egg problem).

- Re-initialize translations after choosing network. There may be a
  different language configured in network-specific settings
  (slim chance, but handle it for sanity).

Fixes point 1 of #3840.
Merge branch 'match_relay_fee' of git://github.com/mikehearn/bitcoin
Merge pull request #3844

c52c4e5 qt: Make it possible again to specify -testnet in config file (Wladimir J. van der Laan)
Fix missing wallet lock in CWallet::SyncTransaction(..)