IBM-Swift/Kitura#962 Fix SSLService crashes and refactor and add tests

[na-gupta]

Description

• Add state guards for IncomingSocketHandler write() and close() functions
• Make sure we don't close sockets while they're being used
• Refactor tests to extend from KituraNetTest
• Add state properties so test configuration can be changed in one place
• Add test SSL self signed certificates for unit testing
• Make most unit tests use SSLService as it surfaces race condition problems whereas Socket mostly ignores them

Motivation and Context

Kitura/Kitura#959
Kitura/Kitura#962

How Has This Been Tested?

Checklist:

• I have submitted a CLA form
• If applicable, I have updated the documentation accordingly.
• If applicable, I have added tests to cover my changes.

[codecov-io]

Current coverage is 83.65% (diff: 72.72%)

Merging #166 into master will increase coverage by 0.12%

@@             master       #166   diff @@
==========================================
  Files            27         27          
  Lines          2344       2362    +18   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits           1958       1976    +18   
  Misses          386        386          
  Partials          0          0          

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[shmuelk]

I want to see this broken up into three separate PRs:

1. Refactor tests
2. Add SSL tests
3. Fix SSL crashes.

[na-gupta]

Will refactor this into two PRs - one for the tests restructuring and another for the SSL fixes