Update client number in f_serverSyncNClientsAndStop function call

Finalise TC_DSLITE_B4_BF_BV_001
Start implementation of TC_DSLITE_AFTR_BF_BV_001

Add IPv4Packets templates

Further adaptation.

Added template for TC_DSLITE_B4_FRAG_BV_002.

Rename Dhcpv4Msg into DHCPv4Msg (aligned with DCHPv6Msg)

Extracted DHCPv6 definitions to be re-used in multiple sub folders.

And file comment on DSLite ATS