MB-13020 Honor transaction isolation level in iterator
MB-13020 Honor transaction isolation level in iterator

While picking WAL items we must honor transaction's isolation level
and ensure that the latest uncommitted item in any transaction is picked up
if the level is FDB_ISOLATION_READ_UNCOMMITTED and and the latest
uncommitted item of the same transaction is picked up if the isolation
level is FDB_ISOLATION_READ_COMMITTED
Change-Id: I86a93251881029151358c949bc239c877eda4bed

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