Split chunk selection out of LSM merge function.
Have LSM cleanup bloom filters consistently.

There were cases before that old bloom filters could
interfere with creating new chunks.
Fix build warnings re: uninitialized variables.
Fixup per review comment.
Don't reset session random number generations, applications that don't
cache sessions will end up generating bad skiplists because their random
numbers will be the same on every update/insert.
Merge pull request #1371 from wiredtiger/lsm-merge-simplify

Lsm merge simplify
whitespace