Resume - Simultaneous Publishes

[MattBrittan]

When the connection is established to the broker and there are messages queued the library sends them all immediately upon connection. When there are a significant number of queued messages and a cellular link is in use this appears to saturate the link resulting in the connection dropping. In addition this can result in significant numbers of out-of-order packets (not that order is guaranteed when resuming from a store).

Having performed some tests it appears that limiting the number messages in flight can significantly reduce these issues. This appears particularly bad when code is running on low power routers; I run a fair number of Teltonika rut955's and resuming on these after an outage is particularly slow (but resolved when in flight messages reduced).

As such I'll raise a PR to add a new option SetMaxResumePubInFlight that will ensure that only the resume function will only send the specified number of messages simultaneously (it will wait for the relevant ACK's before sending the next one).

Note: It would be nice to have a global MaxInFlight option but I don't currently have time to implement that (it would be a much larger change).

[MattBrittan]

Change merged - will leave this open for a while in case any issues arise (if the option is not set this change should have no impact on usage).

[MattBrittan]

Closing this as I've not had any issues in production (and this change has resolved the issue).