Cleanup the code of the MySQL connector

[MeanSquaredError]

This PR makes the following changes to the PostgreSQL connector code:

• The following classes had their method definitions partly in-class and partly out-of-class:

sqlpp::postgresql::detail::statement_handle_t
sqlpp::postgresql::detail::prepared_statement_handle_t
sqlpp::postgresql::prepared_statement_t
sqlpp::postgresql::::Result
sqlpp::postgresql::bind_result_t
sqlpp::postgresql::connection_base

For the above classes all method definitions were moved in-class.

• Several methods that were defined in-class had superfluous inline specifiers that were removed.
• Removed sqlpp::postgresql::Result::hasError() which was declared but not defined and not used.
• Added/removed a couple of comments.
• Simplified cleanup of native PostgreSQL handles pretty much like we did with MySQL handles.
• Added const qualifiers.
• Removed superfluous namespace qualifiers.
• Changed some variables/methods from CamelCase to snake_case.

The PR has been built with

cmake -B build -DBUILD_POSTGRESQL_CONNECTOR=ON -DBUILD_SQLITE3_CONNECTOR=ON -DBUILD_MYSQL_CONNECTOR=ON -DBUILD_TESTING=ON -DUSE_SYSTEM_DATE=ON -DDEPENDENCY_CHECK=ON

All tests passed successfully.

[rbock]

Thanks for the large-scale cleanup!