Fix more CWallet/CWalletDB layer violations #9256

This is another wallet/walletdb refactoring a.k.a. layer violation cleanup.

This is a stepping stone for...

• wallet manipulation tool (compile walletdb without main, etc.) (something like #8745)
• wallet database abstraction (use a different wallet backend [liblogdb, sqlite] in parallel to bdb) (something like #5686)

Together with #9143, this makes CWalletDB independent from CWallet (main goal of the current refactoring).

What is does...

• Removes the CDB::Verify callback (can be executed independently)
• CWalletDB::LoadWallet and CWalletDB::Recover will map and/or check for records over a callback thus allow a flexible layer setup.

First two commits are (sort of) moveonly.

It makes most sense to review https://github.com/bitcoin/bitcoin/pull/9256/commits/8fea0a065c1d45637e2015c6fdeb2a4665c34864

Fails with the following when setting up the rpc network

Error: Cannot downgrade wallet

  File "/home/travis/build/bitcoin/bitcoin/qa/rpc-tests/test_framework/test_framework.py", line 144, in main
    self.setup_network()
  File "/home/travis/build/bitcoin/bitcoin/build/../qa/rpc-tests/p2p-compactblocks.py", line 127, in setup_network
    ["-debug", "-logtimemicros", "-txindex"]])

Closing for now... maybe someone pick's that up later.