userspace/snmp: add Rx/Tx deltas to snmp

add to snmpd and MIB:
-WR-SWITCH-MIB::ppsiDeltaTxM
-WR-SWITCH-MIB::ppsiDeltaRxM
-WR-SWITCH-MIB::ppsiDeltaTxS
-WR-SWITCH-MIB::ppsiDeltaRxS

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>