Connection Pooling and Refactor

[jscott22]

This PR introduces connection pooling using poolboy. The default configuration is to open 128 connections on application start, with 0 overflow. This options can be configured per the documentation.

This PR also improves documentation and increases its consistency.

The majority of modules have also been refactored to clean up the code base. In addition type specs have been added and verified to address #19. All credo issues have been solved as well. #

[codecov]

Codecov Report

Merging #20 into master will decrease coverage by 1.64%.
The diff coverage is 78.46%.

@@            Coverage Diff            @@
##           master     #20      +/-   ##
=========================================
- Coverage   93.85%   92.2%   -1.65%     
=========================================
  Files          19      20       +1     
  Lines         309     308       -1     
=========================================
- Hits          290     284       -6     
- Misses         19      24       +5

Impacted Files	Coverage Δ
lib/data/chunk_reader.ex	97.22% <ø> (ø)	⬆️
lib/admin/table_admin/gc_rule.ex	100% <ø> (ø)	⬆️
lib/auth.ex	50% <ø> (ø)
lib/admin/table_admin/table.ex	100% <ø> (ø)	⬆️
lib/data/sample_row_keys.ex	66.66% <0%> (+16.66%)	⬆️
lib/data/row_filter.ex	100% <100%> (ø)
lib/data/mutate_row.ex	100% <100%> (ø)	⬆️
lib/data/read_modify_write_row.ex	100% <100%> (ø)	⬆️
lib/data/mutate_rows.ex	100% <100%> (ø)	⬆️
lib/bigtable.ex	100% <100%> (ø)	⬆️
... and 10 more

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