Calculate buffer and message size with Buffer.byteLength, not string length

[boblauer]

The buffer length was being calculated using buffer.length but should be calculated using Buffer.byteLength to account for non-ASCII characters whose length differs from the number of bytes they take up. I was able to get the test that I modified to fail before applying my changes, 9 failures in total but here's the last one:

  9) #buffer
       stream child client
         should never allow buffer to exceed maxBufferSize:

      AssertionError [ERR_ASSERTION]: Buffer size 113 exceeded maxBufferSize 100 after message 9
      + expected - actual

      -false
      +true

      at test/buffer.js:122:20
      at onListening (test/helpers/helpers.js:118:5)
      at createServer (test/helpers/helpers.js:218:5)
      at Context.<anonymous> (test/buffer.js:110:18)
      at process.processImmediate (node:internal/timers:491:21)

I attempted to replace all usages of .length with Buffer.byteLength where buffer size is being calculated, hopefully I didn't miss any.

[boblauer]

This is an unrelated change, but because we know flushQueue was just called, we know that bufferHolder.buffer is the empty string, so the this.bufferHolder.buffer === '' ? '' : '\n' check is unnecessary.