Add undocumented debug API

In discussions with daniel.gottlieb it would be helpful, particularly in the coming weeks, to have an API the user can call to show/dump various parts of the system.

It could take a config arg for the subsystem to dump information:

• open cursors (their state, uri, session, is it positioned or reset)
• open sessions
• open data handles, reference counts, uri/filename
• global txn table
• cache dump
• all information

Additionally, it would be nice to have the ability to set certain locations to give stack traces on certain errors. I.e. in early testing we may be doing serial operations and don't expect a WT_ROLLBACK or EBUSY return. Add code where those are (most likely) returned so that if the debug option is set we can print a stack trace at that time.