It is confusing. For query compaction, it initialized to `always_gc`,
for sstable compaction it is initialized to a lambda calling into
`can_gc()`. This makes understanding the purpose of this member very
confusing.
The real use of this member is to bridge
mutation_partition::compact_and_expire() with can_gc(). This patch
ditches the member and creates the lambda near the call sites instead,
just like the other params to `compact_and_expire()` already are.
can_gc() now also respects _tombstone_gc.is_gc_enabled() instead of just
blindly returning true when in query mode.
With this patch, whether tombstones are collected or not in query mode
is now consistent and controlled by the tombstone_gc_state.
03118a27b8