IPv6 (Mulitprotocol support)

[cmoeller-dx]

This PR gives the parser the ability to read data from IPv6 speaking clients.
It also provides the capabilty for interpeting multiprotocol submessage types (REACH_NLRI and UNREACH_NLRI).
The new code supports ipv6 and ipv4 in (UN-)REACH messages.

The human readable output will look like this:

[BGPMessage UPDATE] - XXX Bytes
|- MAC: 40:a6:b7:03:d1:28 -> 2c:21:72:af:87:c9
|- IP: [1:7f8:0:0:0:5:4242:c09d]:179 -> [1337:7f8:0:200:0:9:0:1]:59380
|- Timestamp: 2020-08-03 18:49:02.753615 (1596480542.753615)
|
|- Update Message Sub-Type: NONE
|- Withdrawn Routes Length: 0 Bytes
|- Total Path Attribute Length: 107 Bytes
|- Path Attributes: 
|--- ORIGIN: IGP
|--- AS_PATH: 1337
|--- MP_REACH_NLRI:
|----- Next Hop:
|----- 1337:4242:0:0:0:925c:0:1
|----- 1337:1212:0:0:911:b33f:7712:67f2
|----- 
|----- NLRI:
|----- 1337:7331::/32

Filtering for MP_(UN)REACH_NLRI messages are currently not supported.

[cmoeller-dx]

merged with private master. changes can be found in PR #42