apply_monotonically() is run with reclaim disabled. So with some bad luck,
sentinel insertion might fail with bad_alloc even on a perfectly healthy node.
We can't deal with the failure of sentinel insertion, so this will result in a
crash.
This patch prevents the spurious OOM by reserving some memory (1 LSA segment)
and only making it available right before the critical allocations.
Fixesscylladb/scylladb#19552
(cherry picked from commit 78d6471ce4)
aee0150506