Define property's default value at class level.
Simplify code.

Doc's mention about expiration time is precisely that server's code is
supposed to do just that internally.
Drop unused global and duplicate import.
Simplify pseudo-constants' value.
Drop meaningless default values.

Simplifies code creating a new connection.
Drop TODOs.

Concurrency handling is not required, because a single MemcachedDict should
not be accessed outside a Zope transaction, which is itself bound to a
specific thread.
Pickling a connection is not needed, ERP5Type.Core.MemcachedPlugin is all
we need to serialise.
Factorise class declaration.
Drop dummy method not present on actual implementation.
Initialise MemcacheTool class.
Drop traceback import.
Inline code documentation.

Docstrings are not to annotate what code is doing.
Factorise end-of-transaction cleanup code.
Make SharedDict a new-style class.
Drop duplicate/obvious docstrings.
Stop closing connections explicitly.

Garbage-collecting memcache.Client instance does that automatically.
Document parameters.
Drop unneeded use of thread-local.

memcache module already handle thread-safety, and we actually want to be
transaction-local here, so use a volatile variable instead for caching
needs.
Removes the need to restart Zope for connection parameter changes to take
effect.
Fixes a bad interaction with SelectionTool's code, which causes
transactional isolation to be broken (just for this connector) and too many
network connections to be used.