Stall detection duplicate

[jtduffy]

Duplicate of #1536
Resolves #1527

Some notes:

• Adds a feature which detects slow transactions and reports as SlowTransaction analytic events
• Implementation hooks into transaction listener to maintain map of open transactions. Before harvest, it scans the map of open transactions and identifies the slowest that has been open longer than a threshold. It extracts metadata about that transaction, notably including the stacktrace of the thread that initialized it, and emits an event.
• The implementation has a mechanism preventing the same transaction to be reported multiple times.
• Disabled by default
• Threshold is configurable, and defaults to 1s (i.e. any transaction open for longer than 1s is a candidate to be reported).