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