Handle infinity timeout when awaiting boot finish

[michaelklishin]

Proposed Changes

rabbitmqctl await_startup can fail with an exception that looks like this:

Error:
{:badarith, [{:erlang, :div, [:infinity, 100], []}, {:rabbit, :wait_for_boot_to_finish, 3, [file: 'src/rabbit.erl', line: 787]}, {:rpc, :"-handle_call_call/6-fun-0-", 5, [file: 'rpc.erl', line: 197]}]}

when timeout ends up being infinity in the function that awaits boot completion by periodically checking every N milliseconds.

Spotted by @lukebakken.

Types of Changes

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

[michaelklishin]

Backported to v3.8.x and v3.7.x.