We now create a new header slice instead of taking a `sendLock` as this
is a rare occurrence.

This commit takes a minimal approach to fixing unsafe header re-use
during close and timeout by reallocating the header in those cases.

Partially fixes: coder/coder#2429

Related: #40

This commit fixes an edge case where the `body` passed to
`waitForSendErr` can be written to after returning from the function.
This happens when `sendReady` is buffered on the `sendCh` and the
session is shutdown or the write times out.

When this condition happens and `waitForSendErr` has not yet exited, the
`body` is safely copied into a temporary buffer in `send`. Otherwise
`waitForSendErr` safely created a copy of the `body` and exits, this
essentially results in double buffering for the edge case which seems
acceptable.