Moved individual merge task execution into the scheduler

This change is made in anticipation of scheduling each task using a
specific thread, and required some modification to the interface of
ExtensionStructure. Namely,
    1. ExtensionStructure now supports a get_merge_tasks() interface,
       which returns a list of the individual level merges that would
       need to be performed to complete a buffer flush of specified
       size.
    2. merge_levels and merge_buffer have been promoted to the public
       interface, to allow their use within the scheduler.
    3. merge_buffer has been modified to assume that the structure
       already can support a direct flush of the buffer into L0, it is
       now the responsibility of the caller to ensure that the necessary
       merges have already been completed prior to calling this method.

Currently, preemptive tombstone compactions are non-functional, so some
unit tests are failing. This will be fixed when the thread scheduling
system is set up.

0 files changed, 0 insertions, 0 deletions