issue #139: core: bump CHUNK_SIZE from 16kb to 128Kb

Reduces the number of IO loop iterations required to receive large
messages at a small cost to RAM usage.

Note that when calling read() with a large buffer value like this,
Python must zero-allocate that much RAM. In other words, for even a
single byte received, 128kb of RAM might need to be written.
Consequently CHUNK_SIZE is quite a sensitive value and this might need
further tuning.

mitogen/core.py

@@ -67,7 +67,7 @@
 SHUTDOWN = 105
 LOAD_MODULE = 106
 
-CHUNK_SIZE = 16384
+CHUNK_SIZE = 131072
 _tls = threading.local()