allow user to configure a delay task priority

[d-w-moore]

Provide a configuration variable defining the priority for delayed tasks launched by the indexing plugin. (As of iRODS 4.2.9, delayed task priority will be honored with an execution priority in the range [1,9].)

Motivation: a collection operation can generate many delayed tasks at once. It would be useful to eg. decrease the priority of these, to avoid starving CPU resources for other processes.