When there are many chunkholders and regions, the cost of collecting
and checking tick thread for each one for every region save
becomes the biggest cost for the save call. To avoid this from
happening, collect the chunk holders from the current region's
owned sections.
This showed significant speedup locally when running the
"walk test" found in RegionizedServer locally (>90% of time
was spent on the holder iteration/checking).
While for merging the synchronisation occured, it did not synchronise
for splitting. This resolves a possible CME that may occur while
splitting regions.
It becomes invalid switching dimensions or by moving far. If
it used after teleporting, then it may also trip thread checks.
Fixes https://github.com/PaperMC/Folia/issues/94
Fix two regionizer issues:
In ThreadedRegionizer#addChunk, fix the incorrect handling
of merging two regions where one of the regions had
pending merges. If the first region had pending merges,
and the second was marked as "ready" then the merge would
cause a "ready" region to have pending merges. The fix is
to simply downgrade the "ready" region to "transient,"
as was previously done if the merge was delayed in the
case where the first region was "ticking."
Additionally, prevent the creation of empty regions
by checking if any new sections were created. This would
happen when a section existed, but had no marked chunks
in it AND all of the sections neighbours existed. In these
cases, no region needs to be created as no sections were
created.
