On low core-count machines the request to save could end up being
delayed since all threads are busy doing things like validating
transactions.
So when a really big block came in I ended up throttling while there
was no save method running in parallel at all.
This fixes this and also makes throtteling be a per-thread thing
instead of doing it inside the mutex.
When reorgs greater than 6 are detected we refrain from taking
action (rolling back the chain) and wait for either user interaction
or till the chain extends the chain-tip again.
The current system writes based on the amount of changes, which is a bit
risky as long as we have small blocks as the amount of changes may for
hours be below the "lets write" limit.
So also write every 5 minutes, just to make sure we won't keep data in
memory longer than needed. Also this allows pruning to happen for people
that don't run their node all the time.
we only save a subsection of stuff in memory on each round of saving,
but I set the counter to zero on how long to wait for the next save.
This makes us save more often till we actually saved most stuff, avoiding
buildups of caches until we do the periodic big flush.
This change makes the limits be static variables, which means they are
only settable once for a process. The only usecase so far is to use much
smaller limits in testing situations.
This allows queries like RPC and mempool-accept to continue
even during a prune (we just use the 'old' DB) and when its
done the last active query will delete the old DB.
This assumes a sane filesystem where you can rename a file that
is in use.
TomZ