Lock inside _throttle_base
Lock inside _throttle_base

So if the throttled function is called again, it will wait for the first
call to be completed before checking whether to do it again (so the cached
result can be returned if applicable).

Closes #134

andreasots/lrrbot,mrphlip/lrrbot,mrphlip/lrrbot,andreasots/lrrbot,mrphlip/lrrbot,andreasots/lrrbot