Use sortedcontainers instead of blist

This commit drop-in replaces blist with SortedContainers. They are
written in pure python so work with pypy, but perform as good as
native implementations, at least in a couple benchmarks:

http://www.grantjenks.com/docs/sortedcontainers/performance.html

synapse/util/caches/stream_change_cache.py

@@ -16,7 +16,7 @@
 from synapse.util.caches import register_cache, CACHE_SIZE_FACTOR
 
 
-from blist import sorteddict
+from sortedcontainers import SortedDict
 import logging
 
 
@@ -35,7 +35,7 @@ class StreamChangeCache(object):
     def __init__(self, name, current_stream_pos, max_size=10000, prefilled_cache={}):
         self._max_size = int(max_size * CACHE_SIZE_FACTOR)
         self._entity_to_key = {}
-        self._cache = sorteddict()
+        self._cache = SortedDict()
         self._earliest_known_stream_pos = current_stream_pos
         self.name = name
         self.metrics = register_cache(self.name, self._cache)