Improvements in neosearch settings

[ppizarro]

This closes #44 - Option to configure the store default and more improvements in the settings

Neosearch:

• Option to configurate the max number of indices open

Engine:

• Option to configurate the maximum number of open database files
• Option to configurate the size of cached operations before a write batch occurs
• Option to configurate the kvstore to be used

Store:

• Specific options for the kvstore

[codecov-io]

Current coverage is 58.29%

Merging #45 into master will increase coverage by +0.04% as of e719967

@@            master     #45   diff @@
======================================
  Files           22      22       
  Stmts         1689    1640    -49
  Branches         0       0       
  Methods          0       0       
======================================
- Hit            984     956    -28
  Partial          0       0       
+ Missed         705     684    -21

Review entire Coverage Diff as of e719967

---

Uncovered Suggestions

1. +1.03% via ...osearch/cache/lru.go#132...148
2. +1.03% via ...arch/index/filter.go#10...26
3. +0.91% via ...arch/index/filter.go#67...81
4. See 7 more...

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