Incrementing/decrementing a bogus key

[Brad Fitzpatrick]

The memcache protocol for increment says:

  The data for the item is treated as decimal representation of a 64-bit
  unsigned integer. If the current data value does not conform to such a
  representation, the commands behave as if the value were 0.

This bit me recently, unknowingly having a Python pickled (serialized) value
in memcache when I thought I had an integer.  So I was actually doing an
increment on a serialized integer object, which memcache treats as a zero,
not touching the flags, so when you fetch it back out, you try to
deserialize it, but it's no longer a serialized object and "boom".

I propose we change this edge case of the protocol and make "incr/decr"
return an error (either NOT_FOUND or NOT_NUMBER) if the value wasn't a
decimal representation of a 64-bit unsigned integer.  That is, the value
must effectively match /\s*\d+\s*/.  (not proposing we add a regex library
dependency...)

Thoughts, objections?

Brad
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