Hold notifications when another operation is executing

[cpswan]

Is your feature request related to a problem? Please describe.

In microcontroller environments TLS sockets are expensive in terms of memory (e.g. MicroPython needs a 16Kb contiguous block of free RAM to wrap TLS around a socket connection), so it's desirable to share a connection for monitor and other operations; but we don't want notifications to come crashing into the responses expected for other verbs.

Describe the solution you'd like

Decorate the monitor verb with metadata that requests the atServer holds a notification until other in progress operations are completed.

[gkc]

Backlog until #1259 is complete