Add Random eviction policy

[erni27]

This PR:

• adds new eviction policy EvictionPolicyRandom (it evicts the random entry from the cache if max entries limit exceeded)

Partially fixes: #51

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (50ebc73) 95.79% compared to head (7364fb7) 95.59%.

Files	Patch %	Lines
eviction.go	84.61%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
- Coverage   95.79%   95.59%   -0.20%     
==========================================
  Files           6        6              
  Lines         951      977      +26     
==========================================
+ Hits          911      934      +23     
- Misses         35       38       +3     
  Partials        5        5              

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