Fix: replace custom timer logic with standard context deadlines

[cmacq2]

Description

Gets rid of segfaults and leaked child processes when running in streaming mode.

Previously the OpenFAAS watchdog implemented custom logic based on a timer to kill function processes that exceed some exec_timeout. This solution was susceptible to panics and resource leaks:

• SIGSEGV when attempting to invoke Kill() on an invalid *Cmd pointer
• OOMKills in Kubernetes due to what appears to be the watchdog failing to reap child processes before executing new function call invocations.

With this change the entire timer based solution is discarded in favour of standard CommandContext API from the exec package.

Fixes: #138

Motivation and Context

See: #138

• [x] I have raised an issue to propose this change (required)

How Has This Been Tested?

• Built a container image for the customised OpenFAAS watchdog
  • Based on reverting some of the github actions work to recover an older version of the Dockerfile with convenient build commands.
• Updated my own function container image to use the customised OpenFAAS watchdog image as base image
• Deployed a Function resource using the customised function image with Helm.
• Repeated load tests (see #138 ), while monitoring:
  • Pod restarts: watch kubectl get pods -l faas_function=...
  • Resource consumption: watch kubectl top pod -l faas_function=...
• Observed no pod restarts (no more crashes) and steady memory consumption (no more 'step' increases due to leaked memory/child processes)

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.