Retransmit last flight when in finished

[Sean-Der]

When FSM is in finished and sees a handshake message re-send the last flight again.

[Sean-Der]

@hasheddan @at-wat Looking at the spec I believe FINISHED can be simpler? If we see a handshake when in the FINISHED state do a RTX in case the remote lost the last message.

// [RFC6347 Section-4.2.4]
//                      +-----------+
//                +---> | PREPARING | <--------------------+
//                |     +-----------+                      |
//                |           |                            |
//                |           | Buffer next flight         |
//                |           |                            |
//                |          \|/                           |
//                |     +-----------+                      |
//                |     |  SENDING  |<------------------+  | Send
//                |     +-----------+                   |  | HelloRequest
//        Receive |           |                         |  |
//           next |           | Send flight             |  | or
//         flight |  +--------+                         |  |
//                |  |        | Set retransmit timer    |  | Receive
//                |  |       \|/                        |  | HelloRequest
//                |  |  +-----------+                   |  | Send
//                +--)--|  WAITING  |-------------------+  | ClientHello
//                |  |  +-----------+   Timer expires   |  |
//                |  |         |                        |  |
//                |  |         +------------------------+  |
//        Receive |  | Send           Read retransmit      |
//           last |  | last                                |
//         flight |  | flight                              |
//                |  |                                     |
//               \|/\|/                                    |
//            +-----------+                                |
//            | FINISHED  | -------------------------------+
//            +-----------+
//                 |  /|\
//                 |   |
//                 +---+
//              Read retransmit
//           Retransmit last flight

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.08%. Comparing base (602dc71) to head (f356763).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #646 +/- ## ========================================== - Coverage 80.12% 80.08% -0.04% ========================================== Files 101 101 Lines 5353 5328 -25 ========================================== - Hits 4289 4267 -22 - Misses 687 690 +3 + Partials 377 371 -6 ``` | [Flag](https://app.codecov.io/gh/pion/dtls/pull/646/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | Coverage Δ | | |---|---|---| | [go](https://app.codecov.io/gh/pion/dtls/pull/646/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `80.11% <ø> (-0.04%)` | :arrow_down: | | [wasm](https://app.codecov.io/gh/pion/dtls/pull/646/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `63.93% <ø> (-0.06%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#carryforward-flags-in-the-pull-request-comment) to find out more.

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[Sean-Der]

@at-wat @hasheddan I’m gonna merge these tonight! I believe they will have zero impact