Revert the default behavior for eviction worker threads back to having just the main eviction thread.

refs #1158
Check documentation prior to cutting a release.
Fix a few more documentation nits.
Minor cleanup to the log cursor example.
Add a new per-session flag, WT_SESSION_CAN_WAIT, and only use sessions
with that flag, and which are not holding the schema or lsm-tree locks,
are used by the to make block-manager fsync calls based on os_cache_dirty
configurations.

Sessions that take the flag: application checkpoint, checkpoint server,
eviction workers (and eviction primary if there are no eviction workers)
and handle sweep server.

Reference #1152.
Move the primary eviction thread creation next to its session create,
just for simplicity.
Randomly configure between 0 and 5 eviction worker threads.
A couple of simple cleanups:

Allocate session handles directly into the structure memory we'll check
to decide if they need to be closed, that way if the called function
messes up, we still clean up.

Use the allocated session handle instead of the default session handle
when creating on the internal servers, that way we'll get better error
messages if something goes wrong.
WiredTiger config doesn't support setting the eviction max-threads
to 0, don't do that.

Check the snprintf into the config buffers return, in case we overflow
them.
Don't start the primary thread until the worker threads are created,
else the primary might try and start additional threads on its own.
Cut the 2.3.1 release.
Fix ordering of releases in the documentation landing page.
typo
Merge branch 'develop' into session-can-wait
Turn asynchronous flush of leaf pages back on.

Increase the run-time of the wtperf update-checkpoint-btree
update-checkpoint-lsm runners from 2 minutes to 5 minutes.

Reference #1152.
Increase cache size for longer runtime and larger table. #1152
Reduce insert thread count to 1. #1152
Move cache pool configuration later in wiredtiger_open: it can open sessions.
Add a way to create documentation snippets that are reused in multiple places, convert the table of cursor types to use it.
Merge pull request #1161 from wiredtiger/session-can-wait

Add a new per-session flag, WT_SESSION_CAN_WAIT