Minor cleanup in CouchKVStore::compactDBInternal
Minor cleanup in CouchKVStore::compactDBInternal

Use [const] auto for some variables and delay creating them
until they're used.

Change-Id: I4a10d32581abfb6f8fa13ad0c760fc186c5203b4
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/132133
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine