Use fdatasync in preference to fsync, if it is available (it is twice as fast in my testing).

refs #831
When using fsync or fdatasync for log durability, have groups of threads piggyback on a single call.  This moves most of the stalling to one place, so remove other condition variables and spin instead.

refs #831
Now that log syncing with fsync scales better than O_DSYNC, change the default.

refs #831
fix unused variables
Merge pull request #832 from wiredtiger/log-sync-scaling

Improve scalability of durable log writes