remotefilelog: import pruned-down remotefilelog extension from hg-experimental This is remotefilelog as of my recent patches for compatibility with current tip of hg, minus support for old versions of Mercurial and some FB-specific features like their treemanifest extension and fetching linkrev data from a patched phabricator. The file extutil.py moved from hgext3rd to remotefilelog. This is not yet ready to be landed, consider it a preview for now. Planned changes include: * replace lz4 with zstd * rename some capabilities, requirements and wireproto commands to mark them as experimental * consolidate bits of shallowutil with related functions (eg readfile) I'm certainly open to other (small) changes, but my rough mission is to land this largely as-is so we can use it as a model of the functionality we need going forward for lazy-fetching of file contents from a server. # no-check-commit because of a few foo_bar functions Differential Revision: https://phab.mercurial-scm.org/D4782