feat(reactor): freeze detection support for reactors

This fix introduces reactor heartbeat loop to detect frozen reactors,
which is opted by the “diagnostics” feature (not active by default).
New CLI command --diagnose-stack is added to allow dumping stack for
all threads in running I/O agent, which allows stack trace collection
when a frozen reactor is detected.

Signed-off-by: Mikhail Tcymbaliuk <[email protected]>