dtls: refactoring and fix of PMTUD/Established code

There are various issues in the PMTUD code:
- OOO handler wasn't applied to the first packet
  due to unseen code duplication in PMTU code.
- first packet sequence has to be known in case of OOO on it
- bug in losts detection.
- decrease the lost threshold time.
- temporary packet allocation is not efficient.
- code duplication and functional flow not well designed.
- comments needed

This patch fixes all of that.

Change-Id: I93ec71e22f6cb7a66ad9ab0f927d31044966f1e3
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>