Consider changing exact time of expiration in TTLCache

[tkem]

As @XuehaiPan pointed out in #219 and #220, the current way of determining expiration time is somewhat awkward when using small integer TTLs and/or integer-based timers. For example, with ttl=1, an item will expire at timer() + 2 if timer() returns only integer values. Replacing

link.expire < self.__timer()

with

link.expire <= self.__timer()

throughout ttl.py would probably make this behave more intuitively.

This would be a breaking change, but it is probably safe to expect that most application will use the default timer and shouldn't be affected. Adding another option to the TTLCache constructor for specifying expiration behavior has already been dismissed as too complex.
Note that this will fundamentally change the behavior of ttl=0, but that's probably only relevant for unit tests.