Optimise Siddhi MongoDB store query execution

[PrabodDunuwila]

Purpose

• Optimize MongoDB store to execute select clause in DB by pushing the querying to DB level rather than fetch the records to Siddhi level and executing queries at Siddhi level.

Goals

• Selection of stream, store and constants and supports logical operators, inline math operators, comparison operators and 'is null'.
• Group by with functions min, max, avg, sum and count.
• Having, order by, limit and offset.

Approach

• Implement compileSelection method and query method in MongoDBEventTable. And used a visitor for selection named 'MongoSelectExpressionVisitor'.

Release note

Optimise Siddhi MongoDB store query execution

Documentation

N/A as this is optimisation

Automation tests

• Unit tests - 91%
• Integration tests - N/A

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

Test environment

JDK 1.8, MongoDB 3.6, 4.2

[niveathika]

@PrabodDunuwila It seems your commit messages are clear enough, but better follow standards when coming up with commit messages, as I see some common flows as not using Imperative mood.
Follow them from here on out, https://chris.beams.io/posts/git-commit/

[niveathika]

Let's add more information to the PR since it is referred from the release note and the above info is not enough. Try not to remove content heading in PR template when it is major improvement/new feature