Restructured to use async socket code only.

This reduced the amount of code and should
make the next step, adding failure handling,
much easier.

Also changed invalidation protocol to be atomic
(or close enough).

src/ZEO/Invalidator.py

+"""Facility for (roughly) atomically invalidating cache entries.
+
+Note that this is not *really* atomic, but it is close enough.
+"""
+
+import tempfile, cPickle
+
+class Invalidator:
+
+    _d=None
+    
+    def __init__(self, dinvalidate, cinvalidate):
+        self.dinvalidate=dinvalidate
+        self.cinvalidate=cinvalidate
+
+    def begin(self):
+        self._tfile=tempfile.TemporaryFile()
+        self._d=cPickle.Pickler(self._tfile).dump
+
+    def invalidate(self, args):
+        if self._d is None: return
+        for arg in args:
+            self._d(arg)
+
+    def end(self):
+        if self._d is None: return
+        self._d((0,0,0))
+        self._d=None
+        self._tfile.seek(0)
+        load=cPickle.Unpickler(self._tfile).load
+        self._tfile=None
+
+        cinvalidate=self.cinvalidate
+        dinvalidate=self.dinvalidate
+
+        while 1:
+            oid, serial, version = load()
+            if not oid: break
+            cinvalidate(oid, version=version)
+            dinvalidate(oid, version=version)
+