transport: investigate dynamic buffer sizes for the write buffer

[easwars]

This issues addresses bullet point 5 from #5751.

We currently allocate a static buffer which is twice the size configured by the user. This buffer ends up using a bunch of memory, even when not being actively used.

Some options to consider:

• Dynamically tune buffer size based on activity
• Use a sync.Pool and keep a bunch of buffers around and manage the number (and possibly size) of these buffers based on activity

We should be able to use the changes made as part of #5757 to test the effectiveness of these options.