This requires that we not access pcoinsTip in InitBlockIndex's
FlushStateToDisk (so we just skip it until later in AppInitMain)
and the LoadChainTip in LoadBlockIndex (which there is already one
later in AppinitMain, after ReplayBlocks, so skipping it there is
fine).

Includes some simplifications by Suhas Daftuar and Pieter Wuille.

This should fix a very rare travis failure in zapwallettxes, but
is also more correct, as you can currently race
ReacceptWalletTransactions with stop RPC calls to get bitcoind to
(IMO) eroneously return a non-0 exit code.

A rare race condition may trigger while awaiting the body of a message, see
upsteam commit 5ff8eb26371c4dc56f384b2de35bea2d87814779 for details.

This may fix some reported rpc hangs/crashes.

The bug was introduced in 2.1.6-beta, versions before that don't need the
workaround.

This prevents a potential race condition if control flow ends up in
`ShutdownHTTPServer` before the thread gets to `queue->Run()`,
deleting the work queue while workers are still going to use it.

Meant to fix bitcoin#12362.

Signed-off-by: Wladimir J. van der Laan <[email protected]>

This function, which waits for all threads to exit, is no longer needed
now that threads are joined instead.

Signed-off-by: Wladimir J. van der Laan <[email protected]>

The HTTP worker thread counter, as well as the RAII object that was used
to maintain it, is unused now, so can be removed.

Signed-off-by: Wladimir J. van der Laan <[email protected]>

Adaptation from btc@fa5b440971a0dfdd64c1b86748a573fcd7dc65d3