Buffering: Switch to a more general handle caching type

It sort of implemented an MRU cache but just kept track of the most
recent access and checked the one after it, otherwise searching from
the beginning.

Implement a true MRU list of all open handles. Handles of the current
track will tend to stay up front for faster access.

Switch to common linked list functions

Use double-linked lists to have insert, remove and move_handle
operations in O(1)-- no more searching to find the previous handle,
which is very often required.

Smaller code too. :)

Change-Id: I9ae8f3f96d225a5d54b94133f499268007274784

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