pg_prewarm: Add automatic prewarm feature.

pg_prewarm: Add automatic prewarm feature.

Periodically while the server is running, and at shutdown, write out a
list of blocks in shared buffers.  When the server reaches consistency
-- unfortunatey, we can't do it before that point without breaking
things -- reload those blocks into any still-unused shared buffers.

Mithun Cy and Robert Haas, reviewed and tested by Beena Emerson,
Amit Kapila, Jim Nasby, and Rafia Sabih.

Discussion: http://postgr.es/m/CAD__OugubOs1Vy7kgF6xTjmEqTR4CrGAv8w+ZbaY_+MZeitukw@mail.gmail.com