Rollup node: split context less frequently

Context

The MR !11263 (merged) makes the rollup node split the context on every commitment (60 blocks) and creates a lot of Irmin suffix files. Unfortunately these files need to be opened by the rollup node process so it can exceed the OS file descriptor limit. One one experiment on etherlink (on ghostnet), the Irmin GC started failing and wasn't reclaiming old suffix files which made the rollup node crash.

This MR makes the rollup node split its context at most every refutation window, which means that each suffix file will contain the context updates for ~80k blocks, while still allowing the GC to reclaim more space periodically. In case the GC does not behave correctly, it still leaves decades until we exceed the OS limit (e.g. ~40 years for 2048 files on my machine).