MB-36723: Set Writer threads to minimum priority
MB-36723: Set Writer threads to minimum priority

Decrease the priority of Writer threads to lessen their impact on
other threads (esp front-end workers which should be prioritized ahead
of non-critical path Writer tasks (both Flusher and Compaction).

In the case of the "KV : Pillowfight, 80/20 R/W, 256B binary items",
setting the writer threads to low priority increases Max Throughput
from 2.63M to 3.29M op/s. See MB-36827 for full details.

Only doing this for Linux & Windows at present: On macOS
setpriority(PRIO_PROCESS) affects the entire process (unlike Linux
where it's only the current thread), hence calling setpriority() would
be pointless.

Change-Id: Id3aa1e6bcf66ec437dc3b76eb3b985527b89f843
Reviewed-on: http://review.couchbase.org/117651
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