Fix typo

[ci skip]
Add SOCI 3.2.1 contributors

[ci skip]
Update SOCI 3.2.1 release date

[ci skip]
Fix harmless warning about signed/unsigned comparison.

MSVC (and probably other compilers too) gave a warning about comparing size_t
argument of postgresql_vector_into_type_backend::resize() with the expression
that became integer after the change of 29cb6c7e, so make this expression
unsigned again.
Implement session::get_next_sequence_value() for PostgreSQL backend.

Use the same actual implementation as for the ODBC backend when using
PostgreSQL driver, i.e. "select nextval(sequence-name)".
Don't allow exceptions escape from postgresql_statement_backend dtor.

This fixes abnormal program termination if an error happened while executing a
prepared statement: it was then destroyed during stack unwinding and if
deallocating it failed as well, resulting in a call to terminate().

In practice, deallocation seems to always fail with SQL state 25P02 ("current
transaction is aborted, commands ignored until end of transaction block") when
it is executed inside a transaction.