add --flush-solves to rez-memcache

Sometimes you need to flush the memcache (eg when a new package is made by hand). In these cases it usually isn't necessary to flush _everything_, especially package files, which take the most amount of time to reload from disk. Adding a --flush-solves option would correctly update the cache to detect new, non-rez-released packages, without losing all the cached package files.

Start by locating the rez-memcache entry point and reading how its existing flush behavior selects cached data. Add the --flush-solves option so newly hand-created packages are detected without reloading package files, then verify that the targeted solve cache is refreshed while other cached data is retained.

Written by the indexing model from the issue text.