The maximum number of concurrent invocations to allow. Default: 100, except for the
local provider, where the default is 10.
The concurrency limit applies to all invocations of all of the faast functions summed together. It is not a per-function limit. To apply a per-function limit, use throttle(). A value of 0 is equivalent to Infinity. A value of 1 ensures mutually exclusive invocations.