Make it possible to cancel the current request (statement, procedure) from a different connection

Make it possible to cancel the current request (statement, procedure) from another connection (fiber).

Extend FIBER_CANCEL mechanism to support request-cancels. Currently, when you cancel a fiber, FiberCancelException is thrown. Add FIBER_CANCEL_REQUEST flag, which leads to RequestCancelException, which is a child of ClientError, and thus can be caught by iproto.m loop.

Make sure it's not possible to set the flag or react to it in "internal" fibers, i.e. those which run replication, auxiliary tasks.
Do that by making this extension advisory/voluntary, just like the current cancellation mechanism.
Make sure that all fibers which serve client queries actually participate, and those which don't - are not affected.