Allow concurrent activation execution, in some cases

When creating an action, it would be nice to be able to annotate it as `-a concurrent ` to indicate that concurrent activation executions are allowed in the same container, up to some maximum. This would, for example, allow web actions to be more scalable, instead of limited to the number of concurrently running containers. This would only be allowed for actions/users that are repeatedly invoked - parameters may be unique per activation, but the action+user would have to be the same for concurrent execution.

This would complicate:
- log collection (log would include interleaved messages from multiple activations, which may be ok)
- pause/unpause after activation completion (don't pause till there are no more activations running)

The issue names no files or tests. Start by tracing action creation and activation execution, then inspect how log collection and pause/unpause are handled; done means concurrent executions are supported up to the requested maximum without breaking those behaviors.

Written by the indexing model from the issue text.