MOOSDBVar notifications are not immediately propagated to MOOSAsyncCommClient users

[chrismurf]

If:

• You have a MOOS community consisting of only a single client using the AsyncCommClient
• And the client subscribes to DB_UPTIME, and publishes infrequently

Then:

• DB_UPTIME will be added to the MOOSDB's HeldMailMap for the client every time it is updated, but
• The client will not receive anything until any client sends a packet to the MOOSDB, at which point
• All the DB_UPTIME notifications will be received roughly simultaneously, but significantly delayed.

I believe this is because ThreadedCommServer::ProcessClient() only iterates through asynchronous client threads when data is received, and MOOSDBVar changes do not trigger this processing. Thus, poking a variable with uMS or any client transmission will force processing.