add SysV semaphores

[preaction]

SysV semaphores are shared counters. Anyone can increment/decrement them, and everyone gets the result of the increment/decrement. The counter is held and any new member joining the pool gets the same value.

We could also make this hierarchal, like pub/sub.

Use-cases:

• Job complete notification
• Load signalling
• Locking

Downsides:

• Proper use of this requires messages to be a single integer value.
• Patterns for this will want a library
  • The patterns are really short, so this may not be necessary

[preaction]

They absolutely could not be a hierarchy: When decrementing a parent node, which child node gets it?

Wecould make it so everyone who connects is an "increment", and sends a message to everyone else with the current number of connected clients. Disconnecting decrements the counter (and reports that to other clients).