Make HTTP Service Inside the GraphQL Listener an Isolated Object

[ThisaruGuruge]

Purpose

This PR will make the HttpService object in GraphQL listener an isolated object. This will enable the HTTP service to run parallelly.

Fixes: #1386

Examples

N/A

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #243 (ca67810) into master (2dd95b0) will increase coverage by 0.66%.
The diff coverage is 94.28%.

@@             Coverage Diff              @@
##             master     #243      +/-   ##
============================================
+ Coverage     90.99%   91.66%   +0.66%     
- Complexity      213      215       +2     
============================================
  Files            44       44              
  Lines          1822     1835      +13     
  Branches        771      773       +2     
============================================
+ Hits           1658     1682      +24     
+ Misses          150      139      -11     
  Partials         14       14              

Impacted Files	Coverage Δ	Complexity Δ
graphql-ballerina/annotation_processor.bal	100.00% <ø> (ø)	0.00 <0.00> (ø)
graphql-ballerina/records.bal	72.22% <ø> (ø)	0.00 <0.00> (ø)
...ballerina/stdlib/graphql/runtime/schema/Utils.java	83.78% <ø> (-1.22%)	20.00 <0.00> (-1.00)
graphql-ballerina/engine.bal	84.78% <71.42%> (-4.35%)	0.00 <0.00> (ø)
graphql-ballerina/common_utils.bal	96.00% <100.00%> (ø)	0.00 <0.00> (ø)
graphql-ballerina/engine_utils.bal	100.00% <100.00%> (ø)	0.00 <0.00> (ø)
graphql-ballerina/executor_visitor.bal	91.42% <100.00%> (ø)	0.00 <0.00> (ø)
graphql-ballerina/http_service.bal	100.00% <100.00%> (ø)	0.00 <0.00> (ø)
graphql-ballerina/listener.bal	77.50% <100.00%> (+1.82%)	0.00 <0.00> (ø)
graphql-ballerina/validator_visitor.bal	89.04% <100.00%> (+0.20%)	0.00 <0.00> (ø)
... and 4 more

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