Create multiple parts multipart message if payload is big enough.

For example, if you have 2000+ groups, then OFMP_GROUP to
OFPG_ALL returns a big reply, and the message length
overflows uint16_t, where we really need multiple messages.

Signed-off-by: Hiroaki KAWAI <kawai@stratosphere.co.jp>
Create multiple parts multipart message if payload is big enough.

For example, if you have 2000+ groups, then OFMP_GROUP to
OFPG_ALL returns a big reply, and the message length
overflows uint16_t, where we really need multiple messages.

Signed-off-by: Hiroaki KAWAI <kawai@stratosphere.co.jp>
Merge branches 'multi_multipart' and 'multi_multipart' of https://github.com/iHiroakiKawai/trema-edge into multi_multipart
Merge branch 'multi_multipart' of https://github.com/iHiroakiKawai/trema-edge into iHiroakiKawai-multi_multipart