refs #6646 Adding minimal check to verify that m_session is not null in unlink.  Still need to add locks.  Also minor fixing minor copy and paste bug in staging code for #6603.
refs #6646 Adding minimal check to verify that m_session is not null in unlink.  Still need to add locks.  Also minor fixing minor copy and paste bug in staging code for #6603.

tplavcic/Percona-TokuBackup,dbpercona/Percona-TokuBackup,dbpercona/Percona-TokuBackup,tplavcic/Percona-TokuBackup,Jun-Yuan/Percona-TokuBackup,Jun-Yuan/Percona-TokuBackup,guiquanz/Percona-TokuBackup,dbpercona/Percona-TokuBackup,Jun-Yuan/Percona-TokuBackup,kbauskar/Percona-TokuBackup,guiquanz/Percona-TokuBackup,percona/Percona-TokuBackup,tplavcic/Percona-TokuBackup,tplavcic/Percona-TokuBackup,kbauskar/Percona-TokuBackup,Jun-Yuan/Percona-TokuBackup,percona/Percona-TokuBackup,kbauskar/Percona-TokuBackup,guiquanz/Percona-TokuBackup,dbpercona/Percona-TokuBackup,percona/Percona-TokuBackup,guiquanz/Percona-TokuBackup,kbauskar/Percona-TokuBackup,percona/Percona-TokuBackup