feat(NODE-3750): make maxConnecting configurable

[dariakp]

Description

NODE-3750, changes implement the following spec commit: ce242d

What is changing?

• Added maxConnecting as a pool option that is externally configurable as a client option
  • per the spec, maxConnecting <= 0 should warn, so, in line with our warning = error convention, maxConnecting <=0 will error (note that there are two different errors: one from uint validation, and one to handle 0 specifically)
• Synced new cmap test: pool-checkout-custom-maxConnecting-is-enforced
• Updated uri options connection-pool-options test file with new tests and implemented maxConnection option parsing in the test runner
• Updated existing node driver "all options" tests to include maxConnecting - there isn't any complicated option parsing happening, so it didn't seem to require an additional stand-alone test beyond what is covered by the URI spec tests themselves

Is there new documentation needed for these changes?

Yes, in addition to the API docs, we should also update the connection docs page for the node driver since it lists other pool options there.

What is the motivation for this change?

Spec

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: <type>(NODE-xxxx)<!>: <description>
• Changes are covered by tests
• [N/A] New TODOs have a related JIRA ticket

[durran]

Can we sync the yml test as well for uri options? (connection-pool-options.yml)

[dariakp]

@durran good catch, I didn't even realize I missed them

Pulled in the test