Add verbose option that tracks timestamp state and information

It would be useful to have a verbose mode that logs information about timestamps.

There was some code added to output debug messages for use while developing a test case in this commit:
https://github.com/wiredtiger/wiredtiger/pull/3503/commits/316eb9cf4e314674b93aefbba9e99b32a9d708fa

This change requires:

• Plumbing in a new verbose=[timestamp] configuration option
• Adding a function that can output a timestamp in a user friendly format
• Identifying places where it would be useful to track information about timestamps. Some candidates are:

• Where explicit stable and oldest timestamps are set in the API __wt_txn_global_set_timestamp
• When a checkpoint is created at a timestamp
• When the oldest active timestamp is updated in __wt_txn_update_oldest