CCM-17397: Update mesh-poll Timeout Buffer

[gareth-allan]

Description

This PR updates the mesh-poll lambda's MAXIMUM_RUNTIME_MILLISECONDS environment variable so it is now called LAMBDA_TIMEOUT_BUFFER_MILLISECONDS and sets the value to 60000 (it was originally 240000).

Context

The original variable name was confusing, as it reads like it represents the maximum total time the lambda will run for, but it is actually used as a value to compare the remaining time the lambda has to run against and if there is less time left to run no more messages are processes (see processor.py).

The current value appears to have been set based on the erroneous expectation that it represents the total time the lambda runs for (it's set to 4 minutes), but we saw from the recent performance test that it is actually resulting in the lambda running for only 1 minute (the timeout of 5 mins - 4 mins) before declaring it doesn't have enough time left to process messages and then sitting idle for another 4 minutes until the schedule invokes it again.

Setting this value to 60000ms (1 minute) means that the lambda will now actually process messages for 4 minutes, then leave a minute for the mesh-download lambda to acknowledge them before starting processing again.

Validation

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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