Update evictionpool if needed

[soloestoy]

Update evictionPool every time we call freeMemoryIfNeeded,
in case the old candidates is not the best, imagine that:

Time1: we need free memory and populate some keys to evictionPool
like that: A:10,B:9,C:8
Time2: we delete the bestkey A and free enough memory, then keys
in evictionPool are: B:9,C:8
Time3: we need free memory again, and the idle time of B is 3,
but B is already in evctionPool and idle time is 9, so
we take it as the bestkey to delete, but it is not the
correct bestkey.

Also we should update evictionPool in case maxmemory-policy changed,
because LRU, LFU and TTL has different meanings.

BTW, compare to the whole eviction cycle, update evictionPool before the cycle is not a big deal.

[oranagra]

before evicting a key based on the pool, we populate it again.
so possibly something else got pushed in front.
anyway, what is your suggestion (i'm too lazy to read the code)? to update and re-sort the existing items in the pull before population?

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}