Implement retransmit backoff according to 4.2.4.1

[eh-steve]

Description

As per RFC4347 4.2.4.1 (excluding reset after long period of idleness)

Though timer values are the choice of the implementation, mishandling
of the timer can lead to serious congestion problems; for example, if
many instances of a DTLS time out early and retransmit too quickly on
a congested link. Implementations SHOULD use an initial timer value
of 1 second (the minimum defined in RFC 2988 [RFC2988]) and double
the value at each retransmission, up to no less than the RFC 2988
maximum of 60 seconds. Note that we recommend a 1-second timer
rather than the 3-second RFC 2988 default in order to improve latency
for time-sensitive applications. Because DTLS only uses
retransmission for handshake and not dataflow, the effect on
congestion should be minimal.

Implementations SHOULD retain the current timer value until a
transmission without loss occurs, at which time the value may be
reset to the initial value. After a long period of idleness, no less
than 10 times the current timer value, implementations may reset the
timer to the initial value. One situation where this might occur is
when a rehandshake is used after substantial data transfer.

[codecov]

Codecov Report

Attention: Patch coverage is 80.95238% with 4 lines in your changes missing coverage. Please review.

Project coverage is 80.12%. Comparing base (45e16a0) to head (87ca2d5).

Files	Patch %	Lines
handshaker.go	78.94%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
- Coverage   80.13%   80.12%   -0.01%     
==========================================
  Files         101      101              
  Lines        5330     5343      +13     
==========================================
+ Hits         4271     4281      +10     
- Misses        684      685       +1     
- Partials      375      377       +2     

Flag	Coverage Δ
go	80.15% <80.95%> (-0.01%)	⬇️
wasm	63.92% <80.95%> (+0.01%)	⬆️

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[eh-steve]

I can’t really tell from the logs whether the CI is failing due to my changes…

[Sean-Der]

Thank you so much @eh-steve

Sorry it took such a long time to get to these. I am slowly making my way through the backlog