Handshake is now more resilient

[anarthal]

Errors issued by the server for caching_sha2_password users after a fast auth OK packet has been received (e.g. bad database errors) are now reported correctly
Handshake now correctly detects protocol violation errors, like double auth switches

close #469
close #468

[codecov]

Codecov Report

Attention: Patch coverage is 99.39759% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.15%. Comparing base (6839555) to head (40af68a).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ude/boost/mysql/impl/internal/sansio/handshake.hpp	98.87%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #474   +/-   ##
========================================
  Coverage    99.14%   99.15%           
========================================
  Files          142      143    +1     
  Lines         7278     7297   +19     
========================================
+ Hits          7216     7235   +19     
  Misses          62       62           

Files with missing lines	Coverage Δ
include/boost/mysql/client_errc.hpp	100.00% <ø> (ø)
include/boost/mysql/impl/error_categories.ipp	98.78% <100.00%> (+0.03%)	⬆️
...t/mysql/impl/internal/protocol/deserialization.hpp	97.31% <100.00%> (-0.39%)	⬇️
...ost/mysql/impl/internal/protocol/static_buffer.hpp	100.00% <100.00%> (ø)
...sql/impl/internal/sansio/caching_sha2_password.hpp	100.00% <100.00%> (ø)
...sql/impl/internal/sansio/connection_state_data.hpp	100.00% <100.00%> (ø)
...sql/impl/internal/sansio/mysql_native_password.hpp	100.00% <100.00%> (ø)
include/boost/mysql/impl/is_fatal_error.ipp	100.00% <100.00%> (ø)
...ude/boost/mysql/impl/internal/sansio/handshake.hpp	99.23% <98.87%> (+0.95%)	⬆️

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