MDEV-20844 RBR from binary(16) to inet6 fails with error 171: The event was corrupt, leading to illegal data being read

This patch changes the way how INET6 is packed to the RBR binary log:
- from fixed length 16 bytes
- to BINARY(16) compatible variable length style
  with trailing 0x00 byte compression.

This is to make INET6 fully compatible with BINARY(16) in RBR binary logs,
so RBR replication works in this scenarios:

- Old master BINARY(16) -> New slave INET6
- New master INET6      -> Old slave BINARY(16)

A new class StringPack was added to share the code between
Field_string and Field_inet6.