* Track the mapped port for each NAT mapping transport
Each transport needs to be tracked separately so that a failed mapping request
doesn't cause the listen port for a socket to be cleared to zero.
* add template specialization to allow enums as indexes
* use aux::array for port mappings
Concerning type traits, the C++ standard states: "The behavior of a program
that adds specializations for any of the templates defined in this subclause is
undefined unless otherwise specified."
There is no exception for is_integral.
* introduce a prio_index_t type for indexing into the priority sorted piece list, m_pieces, in piece_picker
* some piece picker simplification
* fix msvc warning for prio_index_t
arvidn