Replacing Cache.lock with sync.RWMutex (instead of sync.Mutex)

[Eifoen]

• replaced type of Cache.lock
• changed Get() calls from lock.Lock() to lock.RLock() (and lock.RUnlock() respectively)
• allows for multiple concurrent readers
• only locks for single process/thread if a write call happens

This should improve performance in excessive concurrent use cases

[codecov-commenter]

Codecov Report

Merging #4 into master will decrease coverage by 6.43%. The diff coverage is 51.42%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
- Coverage   83.43%   77.00%   -6.44%     
==========================================
  Files           6        6              
  Lines         163      187      +24     
==========================================
+ Hits          136      144       +8     
- Misses         17       33      +16     
  Partials       10       10              

Impacted Files	Coverage Δ
cache/cache.go	100.00% <ø> (ø)
cache/cache_data.go	78.50% <51.42%> (-13.07%)	:arrow_down:

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