extend std::exception logging in txdb.cpp
Explicitly ensure that wallet is unlocked in `importprivkey`

This makes for a more useful error reply (fixes #957).
Merge pull request #3457

d78f35a Explicitly ensure that wallet is unlocked in `importprivkey` (Wladimir J. van der Laan)
Merge pull request #3453

96e5f61 extend std::exception logging in txdb.cpp (Philip Kaufmann)
Move only: extract WriteChainState and UpdatedTip from SetBestChain.
Add ConnectTip and DisconnectTip.
Prepare block connection logic for headers-first.

This changes the block processing logic from "try to atomically switch
to a new block" to a continuous "(dis)connect a block, aiming for the
assumed best chain".

This means the smallest atomic operations on the chainstate become
individual block connections or disconnections, instead of entire
reorganizations. It may mean that we try to reorganize to one block,
fail, and rereorganize again to the old block. This is slower, but
doesn't require unbounded RAM.

It also means that a ConnectBlock which fails may be no longer called
from the ProcessBlock which knows which node sent it. To deal with that,
a mapBlockSource is kept, and invalid blocks cause asynchronous "reject"
messages and banning (if necessary).