Fix error when building docs.
MOTOR-45 Test MotorPool.resolve's stack context handling.

Demonstrates that a stack context leak prevents errors from
properly propagating to the caller after name resolution.
MOTOR-45 Fix stack-context leak in MotorPool.resolve.

Whenever MotorPool did DNS lookup, "resolve" installed an exception handler
that remained active for all callbacks in the same context. The handler should
have been cleared once name-resolution succeeded, but it wasn't.

This caused an obvious error and a subtle one: the obvious error was that an
exception thrown in a later callback would peg the CPU at 100% and cause
unbounded memory growth. This is because the error would be caught in the
handler that "resolve" registered, and the handler would try to throw the error
into a dead greenlet. The dead greenlet immediately re-raised the error into
the same stack context, causing the handler to raise it again and so on. Each
trip through the handler added a frame to the traceback.

The subtler error was that any ExceptionStackContext surrounding a call into
Motor would not receive the errors it ought to, because an interviewing context
created by "resolve" might catch all subsequent errors instead of allowing them
to propagate.