Before this change messages were sent serially across sessions but
ordering is important only within a session. This leads to head of
line blocking when a session is slow to send, and also enforcement of
send buffer size and time limits is precluded because it happens at
a lower level in the transport.
This change ensures messages are held up only if there is another
from the same session is being sent. This allows messages from each
session to flow independent of other.
See gh-25581
f5c287a6e6