[FEATURE REQUEST] Parallelize initial connections to devices when using Deltaproxy

[ggiesen]

Is your feature request related to a problem? Please describe.
Currently when starting the deltaproxy process, it connects to each device in sequence, waiting for commands to be executed on the device, before starting the connection to any other devices. When connecting to slower network devices, even with a modest number of devices, this can cause process startup to take many 10's of minutes to complete initialization and be ready to accept commands.

Describe the solution you'd like
Initialize connections in parallel (perhaps make it the number of parallel connections configurable), to speed up initial connection setup.

Describe alternatives you've considered
Run proxy minions instead at a cost of huge amounts of memory, or run more deltaproxy processes with fewer devices assigned per-process.

Additional context
Previously discussed with @garethgreenaway here.