Backends: counter overflow in increment() behaves differently on Redis, Memcached and memory

[allen0099]

Found during the docs audit (#359). The docs now describe the per-backend ranges; the behaviour at the edge still differs.

Backend	Incrementing past the range
Redis	raises a raw redis.exceptions.ResponseError ("increment or decrement would overflow"). backends/redis.py:435-439 only converts the "not an integer" error to CacheXError
Memcached	wraps around silently: counter_entry(2**64 - 1) incremented by 1 returns 0
Memory	grows past 64 bits without limit

validate_delta already limits delta to signed 64-bit, but the resulting value is unchecked.

Proposal

Make overflow one documented behaviour on every backend, most likely CacheXError (Redis: also convert the overflow ResponseError; Memcached: detect the wrap, e.g. compare against the previous value, or document unsigned 64-bit as its range and reject values that would wrap; Memory: bound to signed 64-bit). Decide whether this is a fix for a patch release or a behaviour change for 0.4.0. Tests per backend.