Also mmap cfs files for hybridfs

[danielmitterdorfer]

With this commit we add the .cfs file extension to the list of file
types that are memory-mapped by hybridfs. .cfs files combine all files
of a Lucene segment into a single file in order to save file handles. As
this strategy is only used for "small" segments (less than 10% of the
shard size), it is benefical to memory-map them instead of accessing
them via NIO.

Relates #36668

[elasticmachine]

Pinging @elastic/es-distributed

[danielmitterdorfer]

I have run several experiments to judge the impact of this change. A baseline benchmark with mmapfs for a workload with external ids on a "small" index with roughly 3GB (on a system with 32GB RAM), we see a median throughput of 68700 docs/s. Without this change, hybridfs results in 56900 docs/s (a reduction of 17%). With this change, we are again on par (as expected).

I also ran an update-heavy workload with 40% id conflicts on a larger index of 75GB on a system that only has 8GB available page cache. The baseline (mmapfs) results in 12000 docs/s median indexing throughput whereas hybridfs with this change results in 21800 docs/s.

[danielmitterdorfer]

Backports:

• 7.x: Also mmap cfs files for hybridfs (#38940) #38947
• 7.0: Also mmap cfs files for hybridfs (#38940) #38948
• 6.7: Also mmap cfs files for hybridfs (#38940) #40189