Utilize min heap for keysWithExpiry

[osteensco]

Problem:
We currently use heaps for our LFU and LRU eviction rules. We do not use a heap with simple TTL.
In the evictKeysWithExpiredTTL method, the key eviction process is:

• based on sample size, iterate through the keysWithExpiry map sampleSize times and pseudo random keys to an array
• iterate through this array, check if the keys expired, and delete the key
  This process also has the potential to keep live what should be evicted keys.

Proposed Solution:
Utilizing a min heap instead would simplify this process and allow it to scale more efficiently.

[kelvinmwinuka]

I like this idea. If you're interested in implementing this, please go ahead.

[osteensco]

no problem, I'll work on this.