MB-13955: avoid looping on random picks for bcache_evicts
MB-13955: avoid looping on random picks for bcache_evicts

For the second chance algorithm if we loop on the pick from a
random shard, the loop could end up in an infinite loop.
Instead serially examine all shards if empty.
Change-Id: If9635c9dd330a1cb5e95a2299a5d33f4ce7de40d

zhaog/forestdb,hisundar/forestdb,wurikiji/forestdb,jinxuan/forestdb,nvoron23/forestdb,yyangpan/forestdb,jinxuan/forestdb,hisundar/forestdb,seem-sky/forestdb,nvoron23/forestdb,wurikiji/forestdb,couchbase/forestdb,joliss/forestdb,seem-sky/forestdb,couchbase/forestdb,yyangpan/forestdb,sushmaad/forestdb,zhaog/forestdb,sushmaad/forestdb,joliss/forestdb