(COMMON): Basic (Least Recently Used) LRU caching system

[xtda]

PR Description

This PR adds a basic least recently used cache system that will be used as part of #409 (and others parts if people find it useful)

But as I learned my lesson with #383 and its 15,000 changes I decided to break it up into smaller PR's for additional non-directly related functionality

LRU is a type of caching style that stores elements and moves the most recently accessed to the front and disgards the least used if it would grow beyond capacity

There is both a concurrent safe( with mutex)

func New(capacity uint64) *LRUCache {
	return &LRUCache{
		lru: NewLRUCache(capacity),
	}
}

or called directly for non-concurrent safe version:

func NewLRUCache(capacity uint64) *LRU {
	return &LRU{
		Cap:   capacity,
		List:  list.New(),
		Items: make(map[interface{}]*list.Element),
	}
}

Obviously up to user to decide on requirements as if they need it to be goroutine safe or not and performance of mutexs

Type of change

Please delete options that are not relevant and add an x in [] as item is complete.

• New feature (non-breaking change which adds functionality)

How has this been tested

• go test ./... -race
• golangci-lint run

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation and regenerated documentation via the documentation tool
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally and on Travis with my changes
• Any dependent changes have been merged and published in downstream modules

[xtda]

Also I have not added the ability to "Peek" (get a keys value without moving it to the front of the cache) or List all entires as I personally feel this goes against the point of of a LRU cache as you are accessing it and it was recently used

Happy to add if people want this functionality

[codecov]

Codecov Report

Merging #420 into master will increase coverage by 0.21%.
The diff coverage is 43.81%.

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
+ Coverage   41.12%   41.33%   +0.21%     
==========================================
  Files         164      182      +18     
  Lines       38678    41950    +3272     
==========================================
+ Hits        15906    17340    +1434     
- Misses      21639    23167    +1528     
- Partials     1133     1443     +310

Impacted Files	Coverage Δ
engine/orders.go	0% <ø> (ø)	⬆️
logger/logger_types.go	100% <ø> (ø)	⬆️
config/config_types.go	79.41% <ø> (ø)	⬆️
engine/gctscript.go	0% <0%> (ø)
database/models/postgres/script_execution.go	0% <0%> (ø)
engine/rpcserver.go	0% <0%> (ø)	⬆️
engine/engine.go	0% <0%> (ø)	⬆️
common/cache/cache.go	100% <100%> (ø)	⬆️
gctscript/wrappers/validator/validator.go	100% <100%> (ø)
gctscript/vm/vm_error.go	100% <100%> (ø)
... and 40 more

---

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[thrasher-]

Awesome stuff! Great first caching algo with many more to come (FIFO/LIFO etc). Just a few minor nits

[shazbert]

I have no nits, except what Thrasher has suggested. 👍

[thrasher-]

Thanks for making those changes!

[gloriousCode]

So nice and clean. Very nice 🎉 Just a few minor observations and requests.

[gloriousCode]

Thanks for making those changes! Also confirmed it works concurrently. Looking forward to seeing its use

[thrasher-]

Last minor nits just associated with comments to keep golint happy and to match them up

[thrasher-]

// Contains

[thrasher-]

// ContainsOrAdd checks if cache contains key and if not, adds it to the cache

[thrasher-]

This returns the newest k,v from the LRU

[shazbert]

This is an awesome addition well done!