Fix unable to capture changes while concurrent writes

[lasanthaS]

Purpose

When concurrent/out-of-order writes for the auto-increment column occurs in the database, the source cannot capture such events in the polling mode.

Resolves #31

Goals

This PR introduces a strategy to handle such events by waiting on missing records.

Approach

When the wait.on.missed.record parameter is set to true, the source expects polling.column values in (numeric) sequential order. For each record, the source generates an event. When reading and processing records in batches, if the source identifies a missed record it pauses the processing and waits until the missing record appears. If the missing record does not appear for missed.record.waiting.timeout time period (in seconds), it ignores the record and resumes processing the rest.

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