repoview: add caching bits Add a caching infrastructure to cache hidden changesets. The cache tries to read the cache lazily and falls back to recomputing if no wlock can be obtain. To validate the cache we store a sha of the obstore content and repo heads in the beginning of the cache which we check every request.

repoview: split _gethiddenblockers Split up _gethiddenblockers into two categories: (1) "static' blockers that solely rely on the contents of obstore and are visible children of hidden changsets. (2) "dynamic" blockers, appearing by having wd parents, bookmarks or tags pointing to hidden changesets. We assume that (1) doesn't change often and can be easily cached with a good invalidation strategy. (2) change often, but barely produce blockers, so we can recompute them if necessary.

repoview: use set for blockers Blockers should be unique but tags and bookmarks could point to the same rev, therefore use a set to ensure that we don't have duplicates.