ldmsd should enforce plugin API semantics

[morrone]

ldmsd should enforce the sematics chosen for the plugin API.

For instance, one rule in ldms is that every sampler plugin must have config() called before the sampler is started. It should be fairly simple to add a flag somewhere like struct ldms_plugin_cfg that tracks when config() has been successfully called. Then ldmsd can issue an error any time a command is issued to start a plugin that has not been configured.

Right now ldmsd fully permits starting a plugin without configuring it first.

[morrone]

Not addressed, as far as I can tell.