Use options pattern instead of config

[philipch07]

Description

Uses the options pattern instead of the config pattern, which is a necessary change before adding SNAP (#449).

Reference issue

Resolves #446

[JoTurk]

yeah this is a good direction, we need to add more validations tho.

[codecov]

Codecov Report

❌ Patch coverage is 51.75097% with 124 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.54%. Comparing base (473ed5a) to head (f71b4fd).

Files with missing lines	Patch %	Lines
association_options.go	8.82%	61 Missing and 1 partial ⚠️
association.go	77.91%	19 Missing and 17 partials ⚠️
association_rack_options.go	0.00%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #450      +/-   ##
==========================================
- Coverage   83.86%   81.54%   -2.33%     
==========================================
  Files          52       54       +2     
  Lines        4060     4258     +198     
==========================================
+ Hits         3405     3472      +67     
- Misses        479      590     +111     
- Partials      176      196      +20     

Flag	Coverage Δ
go	81.54% <51.75%> (-2.33%)	⬇️

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[JoTurk]

looks good, i think we'll just need to add test coverage and make newAssociationWithOptions public

[JoTurk]

^ mean making newAssociationWithOptions through new APIs to construct Client and Server.

[JoTurk]

looks good, just need to finish it, fix the lint and it's ready for merge.

[philipch07]

^ mean making newAssociationWithOptions through new APIs to construct Client and Server.

@JoTurk I'm not sure if the client and server need to have specific settings for each beyond the existing options. Did you have something more specific in mind? I left a couple TODO's that you can ctrl+f in associations.go and association_options.go where I'm unsure about how to proceed.

Also looking into createAssociation(), it currently just takes Config but changing to ServerConfig or ClientConfig would imply to users that there are different configs that need to be set up, so we would probably have to be careful to specify whatever the differences are if we choose to split them.

[JoTurk]

@philipch07 In DTLS we have Option which is a union between client and server options. For SCTP we don't need this split now because our current options symmetric, but IMO it's nice to split them early before we eventually add dial/listen APIs in v2 and different options for client and server with overlapping options.

100% up to you, I support any choice you pick.