Replace two thread modes (individual insert, read & update threads,
and "job-mix" threads) with a single configuration for a thread
count and workload expressed as a ratio of inserts, reads and updates.
Ref #775.
The rest of the changes for #775: we can now support multiple, entirely
unrelated workloads.
Merge branch 'develop' into thread-group
Update the canned configurations inside of the wtperf source code.
Update the runners again: now that threads takes groups of options,
you can't do:

	threads=(count=1,read=1)

you have to do:

	threads=((count=1,read=1))

instead, doubling the parentheses.
Michael notes the runner scripts changes were wrong, we need to have
dedicated threads for inserts, reads, and updates, that's how they
currently work.  #783
Merge pull request #783 from wiredtiger/thread-group

Add thread group configuration to wiperf.