thread_pool: replace specific accounting for @spawned by @workers.size
thread_pool: replace specific accounting for @spawned by @workers.size

The two were always in sync, so there's really no point in keeping
both. In addition, the accessors are replaced by spawned_threads
and waiting_threads (instead of only spawned) to explicitly identify
the worker-related counters (waiting, running, ...) from the
thread-related counters.

orocos-toolchain/utilrb-ext,orocos-toolchain/utilrb-ext