Improve FlowLock API

FlowLock needs some updates.

* `FlowLock::take()` should return a `FlowLock::Releaser` rather than forcing a client to instantiate one immediately afterwards.
* `FlowLock::take()` should have its arguments as `(int64_t amount, int TaskID)` rather than the other way around
* Renaming `FlowLock` to something like `FlowLimiter` or `FlowSemaphore` or some name that implies that it's more than a mutex would be great.