Improve and simplify the LSM switch logic: it is fine to keep writing into a chunk while it is being switched, until either it hits the hard chunk size limit or a switch transaction ID is chosen that is larger than the writer's.  Fixes an assertion failure introduced by #1432 that could write to an old chunk after the switch completed.

refs #1432, #1418
Don't try to find a chunk to flush in empty LSM trees.
Take more care to only set NEED_SWITCH once when a chunk becomes full.
Make sure that the LSM tree is set up to trigger a switch before looping in __clsm_enter_update.
Merge branch 'develop' into lsm-switch-simplify
Clarify the use of randomness in chunk selection for flushes.
Deal with the case where there is no chunk in an LSM tree to flush.
Implement @agorrod's review feedback.
Merge pull request #1449 from wiredtiger/lsm-switch-simplify

Improve and simplify the LSM switch logic