Don't reuse any curl handles for solo mining and break out of the lp thread if the pool is removed.
Toss out the curl handle after each solo poll
One less block detection message
Only start block detection with gbt solo if setup succeeded
Use the one curl instance for all gbt solo operations, protecting its use with a bool set under gbt lock.
Provide a cg_uilock to unlock the intermediate variant of cglocks.
Use upgrade cglock variants in get_gbt_curl