Switch log code from busy wait loops to condition variables.

This helps with highly multi-threaded workloads.
Merge branch 'fine-grained-durability' into log-condition-variables
Don't use condition variables in log code when no-sync is configured.

Also add some statistics to the log slot code.
Merge pull request #758 from wiredtiger/log-condition-variables

Switch log code from busy wait loops to condition variables.