Proof: improve dynamic startup workflow
Proof: improve dynamic startup workflow

The current implementation of dynamic worker startup requires at least one worker available to launch a query.
This patch allows to submit a query even without workers and to have it automatically start processing when the
first workers comes. Required for PoD setups using resource management systems with potentially large startup
latency, e.g. PanDa.
The time the master waits for the first worker to come is controlled by the ROOT-rc

       Proof.DynamicStartupTimeout 600

(value in seconds). Default is infinite.

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