Fix data race in AssertLogContains and functional race in TestDocChangedLogging

[bbrks]

• Ensures TestDocChangedLogging waits for a marker doc to be recieved, as we can't reliably wait for the no-op of non-metadata doc write to happen to assert on logs.
• Forces console log buffer flush before checking contents, instead of retry loop.
• Avoids multiple buffer reads during log writes
  • Prevents race if the log write causes buffer to grow during read inside retry loop
  • Prevents functional issues if buffer read was in the middle of the log line causing failed assertions (log lines split over multiple buffer reads)

Reliably tested before/after fix with:

$ go test -run='^TestDocChangedLogging$' -race -count=1000 -v ./rest/changestest

Integration Tests

• n/a

[bbrks]

... this made the test fail way more reliably without a data race 🤔

There's something else going on here that I'd need to investigate