Fix race in WAL and WALSegmentWriter
Fix race in WAL and WALSegmentWriter

WAL currentSegmentWriter was not accessed under a mutex.  The WALSegmentWriter
also did not use a mutex to protect the underlying writer.

mnuessler/influxdb,m4ce/influxdb,samaitra/influxdb,samaitra/influxdb,influxdb/influxdb,pkittenis/influxdb,daachi/influxdb,archsaber/influxdb,pkittenis/influxdb,apigee/influxdb,mvadu/influxdb,n1tr0g/influxdb,jonseymour/influxdb,joaopedrotaveira/influxdb,mark-rushakoff/influxdb,jgeiger/influxdb,n1tr0g/influxdb,sudabon/influxdb,m4ce/influxdb,arussellsaw/influxdb,jonseymour/influxdb,arussellsaw/influxdb,influxdata/influxdb,jonseymour/influxdb,n1tr0g/influxdb,n1tr0g/influxdb,otoolep/influxdb,chenziliang/influxdb,mark-rushakoff/influxdb,benbjohnson/influxdb,m4ce/influxdb,nooproblem/influxdb,chenziliang/influxdb,benbjohnson/influxdb,jipperinbham/influxdb,arussellsaw/influxdb,sudabon/influxdb,linearb/influxdb,mnuessler/influxdb,samaitra/influxdb,linearb/influxdb,otoolep/influxdb,otoolep/influxdb,li-ang/influxdb,nooproblem/influxdb,linearb/influxdb,nooproblem/influxdb,pkittenis/influxdb,runner-mei/influxdb,sudabon/influxdb,otoolep/influxdb,jgeiger/influxdb,benbjohnson/influxdb,apigee/influxdb,jgeiger/influxdb,Mindera/influxdb,Mindera/influxdb,jipperinbham/influxdb,Mindera/influxdb,jipperinbham/influxdb,daachi/influxdb,pkittenis/influxdb,mark-rushakoff/influxdb,daachi/influxdb,n1tr0g/influxdb,chenziliang/influxdb,mvadu/influxdb,samaitra/influxdb,influxdata/influxdb,runner-mei/influxdb,samaitra/influxdb,mark-rushakoff/influxdb,jgeiger/influxdb,apigee/influxdb,vladlopes/influxdb,linearb/influxdb,jipperinbham/influxdb,otoolep/influxdb,joaopedrotaveira/influxdb,jonseymour/influxdb,influxdb/influxdb,jgeiger/influxdb,mnuessler/influxdb,apigee/influxdb,archsaber/influxdb,archsaber/influxdb,daachi/influxdb,mark-rushakoff/influxdb,sudabon/influxdb,archsaber/influxdb,linearb/influxdb,mnuessler/influxdb,Mindera/influxdb,jgeiger/influxdb,samaitra/influxdb,joaopedrotaveira/influxdb,daachi/influxdb,mvadu/influxdb,n1tr0g/influxdb,jonseymour/influxdb,sudabon/influxdb,influxdb/influxdb,chenziliang/influxdb,li-ang/influxdb,joaopedrotaveira/influxdb,runner-mei/influxdb,vladlopes/influxdb,nooproblem/influxdb,apigee/influxdb,m4ce/influxdb,mnuessler/influxdb,pkittenis/influxdb,mnuessler/influxdb,arussellsaw/influxdb,influxdb/influxdb,sudabon/influxdb,jonseymour/influxdb,influxdb/influxdb,runner-mei/influxdb,m4ce/influxdb,benbjohnson/influxdb,apigee/influxdb,influxdb/influxdb,Mindera/influxdb,daachi/influxdb,pkittenis/influxdb,mvadu/influxdb,arussellsaw/influxdb,jipperinbham/influxdb,Mindera/influxdb,otoolep/influxdb,m4ce/influxdb,vladlopes/influxdb,mvadu/influxdb,nooproblem/influxdb,vladlopes/influxdb,mark-rushakoff/influxdb,mvadu/influxdb,joaopedrotaveira/influxdb,jipperinbham/influxdb,arussellsaw/influxdb