Increase merge limit of Blocks segments to 500K

Now blocks and bor files are 100K limited. It's because E2 was not able to release small files on R2. But now we can - so, we can afford bigger merge limit.

It will simplify code: now some chains has 500K limit for old files and 100K for new. (some users
It will improve performance: less shards (in sharded data) - better performance (less bloom filters checks, etc...). For example lookup txn by hash will be faster. (I saw this bottleneck in BSC pprof of trace_transaction)

Need do same for bor files (they are aligned with blocks)

This is not urgent task - let's work on it after 3.1 release.
Probably we don't need any special files change - all snapshotters and user's code will just do +1 merge in background and release naturally.

Also maybe it's good chance to change how `ExistenceFilter` works in recsplit index. I don't know if It make sense to do this features together or not (maybe not).