nodemap: add a (python) index class for persistent nodemap testing Using the persistent nodemap require a compeling performance boost and an existing implementation. The benefit of the persistent nodemap for pure python code is unclear and we don't have a C implementation for it. Yet we would like to actually start testing it in more details and define an API for using that persistent nodemap. We introduce a new `devel` config option to use an index class dedicated to Nodemap Testing. This feature is "pure" only because having using a pure-python index with the `cext` policy proved more difficult than I would like. There is nothing going on in that class for now, but the coming changeset will change that. Differential Revision: https://phab.mercurial-scm.org/D7840

nodemap: delete older raw data file when creating a new ones When we write new full files, it replace an older one with a different name. We add the associated cleanup for the older file to be removed after the transaction. We delete all file matching the expected pattern to give use extra chance to delete orphan files we might have failed to delete earlier. Note: eventually we won't rewrite all data for each transaction. This is coming in later changesets. Differential Revision: https://phab.mercurial-scm.org/D7839

nodemap: use an intermediate "docket" file to carry small metadata This intermediate file will make mmapping, transaction and content validation easier. (Most of this usefulness will arrive gradually in later changeset). In particular it will become very useful to append new data are the end of raw file instead of rewriting on the file on each transaction. See in code comments for details. Differential Revision: https://phab.mercurial-scm.org/D7838