hgweb: use separate repo instances per thread Before this change, multiple threads/requests could share a localrepository instance. This meant that all of localrepository needed to be thread safe. Many bugs have been reported telling us that localrepository isn't actually thread safe. While making localrepository thread safe is a noble cause, it is a lot of work. And there is little gain from doing so. Due to Python's GIL, only 1 thread may be processing Python code at a time. The benefits to multi-threaded servers are marginal. Thread safety would be a lot of work for little gain. So, we're not going to even attempt it. This patch establishes a pool of repos in hgweb. When a request arrives, we obtain the most recently used repository from the pool or create a new one if none is available. When the request has finished, we put that repo back in the pool. We start with a pool size of 1. For servers using a single thread, the pool will only ever be of size 1. For multi-threaded servers, the pool size will grow to the max number of simultaneous requests the server processes. No logic for pruning the pool has been implemented. We assume server operators either limit the number of threads to something they can handle or restart the Mercurial process after a certain amount of requests or time has passed.