UdpSocketHandler&IOStatisticCollector: avoid address -> string conversion

With ENABLE_PER_ADDRESS_TRACKING disabled by default, converting address
to strings is waste of time. Replace key with address/port combination.
(cherry picked from commit c0202f89bc0c96bc993f8089efa97f466840036a)
UdpSocketHandler: update UDP header length for IPv6

Using IPv4 UDP header length results in oversize and packet fragmentation on IPv6 links with MTU 1280.
Raises overhead from 2.24% to 2.27% for IPv4, reduces overhead from 8% to 3.8% (and packet loose probability ~twice) for IPv6 with (real) MTU 1280.
(cherry picked from commit dc0cce58c6d86bdaac66056bcaa0548862e1121d)
UdpSocketHandler: account for receiving packets UDP header too (cherry picked from commit edcce55ecf8c87acdf826316d4cc79a8879bb9d1)
NodeStats: remove leftover variable

Unused since commit 884c4cbd4f3446fe3beab2a0703df5f784ad6270
(cherry picked from commit ad5c5831256e702794a32e47d6747105c598df95)
BulkTransmitter: remove unused variable (cherry picked from commit 6952770e71666b4c07eb8b15204ed493076ff8ec)