MB-25875: Make getMultiFsReadCount into Couchbase::RelaxedAtomic<size_t>

A data race was observed while trying to read the getMultiFsReadCount
between the background fetcher and the statistics snapshot task

WARNING: ThreadSanitizer: data race (pid=9742)
Read of size 8 at 0x7d8800012160 by thread T19:
Previous write of size 8 at 0x7d8800012160 by thread T15:

Change-Id: I0df032c89d11163937cbab9625d6572b193bd7f2
Reviewed-on: http://review.couchbase.org/82926
Reviewed-by: Manu Dhundi <[email protected]>
Tested-by: Build Bot <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>

Author: Sriram Ganesan

engines/ep/src/kvstore.h

@@ -333,7 +333,7 @@ class KVStoreStats {
     Histogram<hrtime_t> snapshotHisto;
 
     // Count and histogram filesystem read()s per getMulti() request
-    size_t getMultiFsReadCount;
+    Couchbase::RelaxedAtomic<size_t> getMultiFsReadCount;
     Histogram<uint32_t> getMultiFsReadHisto;
 
     // Histogram of filesystem read()s per getMulti() request, divided by