Pre-enabler changes to umem_cache_map routine to enable allocator to
mark a non-evictable page as evictable and visa-versa. The allocator
will make use of the above changes as follows:
- When all memory blocks in a non-evictable page is marked unused,
  allocator can mark the entire page as unused and will notify the
  umem_cache to mark the page as evictable. No further allocations
  from this memory block will happen from now ownwards.
- Allocator will request the umem_cache to map the page again as
  non-evictable when there is a need to extend non-evictable region
  or as evictable page when a new evictable page has to be reserved.

Signed-off-by: Sherin T George <[email protected]>

Addressed review comments on mem.c

Signed-off-by: Sherin T George <[email protected]>

Enhanced the DAV_V2 allocator to support evicting empty
non-evictable memory buckets from umem cache post a DAOS GC or
aggregation. This inturn enables more evictable memory buckets to
be cached.

Signed-off-by: Sherin T George <[email protected]>

Address review comment on checking the env variable during pool
open/create and fixed valgrind related failure.

Signed-off-by: Sherin T George <[email protected]>

Inorder to ensure that gc happens in an incremental fashion, garbage
reclaim is triggerred not only when number of empty buckets is greater
than threshold but also when free space in general has increased by a
threshhold.

Signed-off-by: Sherin T George <[email protected]>