Add WT_COLLATOR::customize, allowing collators to be customized for specific data sources, or with application-managed metadata.
Add upgrading notes about the WT_COLLATOR change.
Merge pull request #1165 from wiredtiger/custom-collators

Add WT_COLLATOR::customize, allowing collators to be customized
Clarify the comment when clearing the eviction walk point: that can cause eviction, revert the flag.

refs #1160