feat(NODE-3697): reduce serverSession allocation

[nbbeeken]

Description

What is changing?

• incrementTransactionNumber will no longer acquire a serverSession because it won't use the getter

What is the motivation for this change?

With this change our driver will only acquire a serverSession when the getter is used which is in applySession, used inside the Connection class. This limits the number of serverSession allocated to be bounded by at least the number of concurrent operations running and at best by the max number of connection in our connectionPool.

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
• New TODOs have a related JIRA ticket

[dariakp]

I'm sorry I haven't gotten around to a review yet, but it's on my list to do Monday!

[dariakp]

Much easier to follow the tests now, thanks! I resolved the comments that have been addressed, there are 2 comments with active discussions left, and I noticed a couple more nits here:

[dariakp]

LGTM assuming CI passes!

Bailey LGTMed given pipeline pass