revlog: Extract low-level random-access file read caching logic
The `revlog` class does many things, among which fulfilling requests for
arbitrary byte slices from the revlog "data file" by reading a larger chunk
and caching it in memory, in order to reduce the number of system calls.
This extracts that logic into a new class, so that it may later also be used
for the side-data file (with another instance of that class).
The copyright notice of the new file does not include a date or author name
since such information tend not to be kept up-to-date:
https://www.linuxfoundation.org/en/blog/copyright-notices-in-open-source-software-projects/
Differential Revision: https://phab.mercurial-scm.org/D10878
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