Identify stale WebSockets

[ckeshava]

High Level Overview of Change

This PR fixes #390.
Every message transmission through a Websocket is used as its heartbeat signal. If a Websocket has not communicated in the last one minute, then it has failed to transmit ~20 validated ledgers in the ensuing downtime. Such Websockets are categorised as stale and removed from the connection_health table.

Context of Change

Type of Change

• Bug fix (non-breaking change which fixes an issue)
  None of the APIs have been modified as a part of this PR.
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (You added tests for code that already exists, or your new feature included in this PR)
• Documentation Updates
• Release

Test Plan