Decouple connection pool from HTTP client

[arukshani]

Purpose

Is to decouple the connection pool from HTTP client.

Approach

By default HTTP clients will share a global pool but they also has a way to configure their own pool

Automation tests

• Unit tests Added

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes
• Ran FindSecurityBugs plugin and verified report? yes
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes

[codecov-io]

Codecov Report

Merging #13355 into master will increase coverage by <.01%.
The diff coverage is 81.48%.

@@             Coverage Diff              @@
##             master   #13355      +/-   ##
============================================
+ Coverage     63.98%   63.99%   +<.01%     
  Complexity      659      659              
============================================
  Files          2147     2148       +1     
  Lines         94210    94305      +95     
  Branches      12406    12414       +8     
============================================
+ Hits          60281    60350      +69     
- Misses        29035    29059      +24     
- Partials       4894     4896       +2

Impacted Files	Coverage Δ	Complexity Δ
...nalang/net/http/clientendpoint/InitGlobalPool.java	100% <100%> (ø)	0 <0> (?)
...main/java/org/ballerinalang/net/http/HttpUtil.java	72.32% <74.54%> (-1.03%)	0 <0> (ø)
...et/http/clientendpoint/CreateSimpleHttpClient.java	88.63% <94.44%> (+2.92%)	0 <0> (ø)	⬇️
...allerinalang/net/jms/LoggingExceptionListener.java	50% <0%> (-50%)	0% <0%> (ø)
...rinalang/runtime/threadpool/ThreadPoolFactory.java	61.9% <0%> (-4.77%)	0% <0%> (ø)
...a/org/ballerinalang/bre/bvm/WorkerDataChannel.java	83.95% <0%> (-4.33%)	0% <0%> (ø)
...mantics/analyzer/StreamsQuerySemanticAnalyzer.java	87.47% <0%> (+0.13%)	0% <0%> (ø)	⬇️
...inalang/compiler/desugar/StreamingCodeDesugar.java	96.73% <0%> (+0.35%)	0% <0%> (ø)	⬇️
... and 4 more

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