In practice this means that CheckBlock+ContextualCheckBlock are
called with a passed-in CValidationState before we move onto
connecting the best chain. This makes conceptual sense as these
calls represent the DoS checks on a block (ie PoW and malleability)
which the caller almost certainly wants to know about right away
and shouldn't have to wait on a callback for (and other
validationinterface clients shouldn't care about someone submitting
bogus malleated blocks to PNB).

This also makes it much, much easier to move the best chain
activation logic to a background thread as it implies that if PNB
returns with a IsValid() CValidationState we don't need to care
about trying to process (non-malleated) copies of the block from
other peers.

This prepares for making best-chain-activation and disk writes
happen in a separate thread from the caller, even though all
callsites currently block on the return value immediately.

CNodeState was added for validation-state-tracking, and thus,
logically, was protected by cs_main. However, as it has grown to
include non-validation state (taking state from CNode), and as
we've reduced cs_main usage for other unrelated things, CNodeState
is left with lots of cs_main locking in net_processing.

In order to ease transition to something new, this adds only a
dummy CPeerState which is held as a reference for the duration of
message processing.

Note that moving things is somewhat tricky pre validation-thread as
a consistent lockorder must be kept - we can't take a lock on the
new cs_peerstate in anything that's called directly from
validation.

This resolves the performance regression introduced in the previous
commit by always waking the message processing thread after each
block future resolves.

Sadly, this is somewhat awkward - all other validationinterface
callbacks represent an actual change to the global validation state,
whereas this callback indicates only that a call which one
validation "client" made has completed. After going back and forth
for some time I didn't see a materially better way to resolve this
issue, and luckily its a rather simple change, but its far from
ideal. Note that because we absolutely do not want to ever block on
a ProcessNewBlock-returned-future, the callback approach is
critical.

To keep the API the same (and for simplicity of clients, ie
net_processing), this splits AcceptBlock into the do-I-want-this
stage, the checking stage, and the writing stage.

ProcessNewBlock calls the do-I-want-this and checking (ie
malleability checking) stuff, and then dumps blocks that pass
into the background thread. In the background, we re-test the
do-I-want-this logic but skip the checking stuff, before writing
the block to disk and activating the best chain.

As reject messages are required to go out in-order (ie before any
further messages are processed), this sadly requires that we
further delay re-enabling a peer after a block has been processed
by waiting for current validationinterface callbacks to drain.

This commit enables further reduction of cs_main in net_processing
by allowing us to lock cs_peerstate before cs_main in BlockChecked
(ie allows us to move things which are accessed in BlockChecked,
including DoS state and rejects into CPeerState and out of
CNodeState).

This technically resolves a race where entries are added to
mapBlockSource before we know that they're non-malleated and then
removed only after PNB returns, though in practice this wasn't an
issue since all access to mapBlockSource already held cs_peerstate.

This removes the cs_main lock which is taken on every
ProcessMessages call.

This moves one more group of variables out of cs_main. Importantly,
these moves allow us to do block processing almost entirely without
cs_main once mapBlockIndex moves to its own (read) lock.

This has the effect of allowing CheckBlock to run in parallel with
other block connection things in some cases during IBD.