kvcoord: elide locking requests in txnWriteBuffer

[stevendanna]

If we know that a sufficient lock is already held at the given sequence number, we can often elide duplicate locking requests. Currently, the only requests that modify the buffer are write requests.

The test adds skipped cases that would require us to buffer the returned locks and values.

Epic: none Release note: None

[cockroach-teamcity]

This change is 

[stevendanna]

TFTR!

I'm going to merge this one. But if there are any follow up comments, please feel free to leave them. I don't mind doing another PR to fix up issue.

bors r=yuzefovich

[craig[bot]]

Build succeeded:

• linux_amd64_fips_build
• check_generated_code
• acceptance
• examples_orms
• lint
• linux_amd64_build
• local_roachtest
• docker_image_amd64
• unit_tests
• local_roachtest_fips