Support --threads in the cli_args rpc method

[drewbanin]

Describe the feature

Presently, the --threads flag passed when running dbt rpc --threads X will be used for subsequent cli_args tasks submitted to the rpc server. If a cli_args task provides its own --threads flag, this value is discarded. Instead, the --threads flag supplied in a cli_args task should supersede the --threads flag supplied when the rpc server was started.

# Every subsequent cli_args task should use 10 threads
dbt rpc --threads 10

# A request like this should run with 2 threads
POST /rpc
{
  "params": {
    "cli": "run --threads 2"
  }
}