Draft: Update index cache size in lib_context
This MR increases the default size of the internal cache of index to take advantage of the optimisations available with irmin.2.8.0 (see !3679 (merged)).
We extracted this from #3679 (closed) to isolate better the flaky tests that seemed to fail due to this change.
Rebased on !3679 (merged).
Edited by Ioana Cristescu