metadata is now implemented as a single std::vector

[anarthal]

Instead of several separate std::string objects, metadata now uses a single std::vector and offsets, which is more efficient
Default-constructing metadata objects now leaves all fields in a well-defined state
Added unit tests for metadata

close #461
close #465
close #164

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.02%. Comparing base (303b9f0) to head (8f45ade).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #466   +/-   ##
========================================
  Coverage    99.02%   99.02%           
========================================
  Files          142      142           
  Lines         7262     7280   +18     
========================================
+ Hits          7191     7209   +18     
  Misses          71       71           

Files with missing lines	Coverage Δ
include/boost/mysql/metadata.hpp	100.00% <100.00%> (ø)

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