Fix warnings by changing the code whenever possible, in particular remove
firebird_rowid_backend class and src/backends/firebird/row-id.cpp file in
which it was defined entirely as it only resulted in "unreachable code"
warnings but was otherwise unused.

Also avoid implicit conversions by either using the correct types or by making
the casts explicit using static_cast<> in places where the casts are really
needed.

Add helpful SOCI_NOT_COPYABLE, SOCI_NOT_ASSIGNABLE and SOCI_UNUSED macros and
use them to suppress the corresponding warnings.

Finally, combine soci-config.h and soci-platform.h in a single file, always
include the latter as the first header and keep the former as a wrapper for
the latter just for compatibility.

Remove the MSVC "#pragma warning(disable: *)" as they are not needed any more.

Closes #355.