allocation query: Add API to increase and update the minimum require buffers

Submitted by Nicolas Dufresne @ndufresne

Link to original bug (#746212)

Description

As it goes, the minimum buffers in an allocation pool has a special meaning. It represent the minimum amount of buffers required in order for a segment of a pipeline to function. Element that retain buffers must update this value in order to ensure the pool will function.

Unfortunately this value is not exposed globally, but rather per allocation pool. This means that to update/increase this value, one need to iterate over the pool list, remove pool which have a maximum too small and update minimum. In order to make this task easier, we could add utilities in the core API. Currently this is implemented by videorate only, but queues with minimum threshold should implement that too, deinterlace element, and many others may also need to do so.