Add cxy_ctl for spawn limiting from jaynel/circular-buffer

Allow spawning of processes up to a limit, after which new requests
run inline. The spawn counting and caps are kept in an ets table
named 'cxy_ctl'. A ring buffer of execution times is also created
for each concurrency type in the 'ets_buffer' table.

Concurrency limiters are initialized together in a single action
from a proplist of {Task_Type::atom(), Type_Max::pos_integer(),
Timer_History_Count::non_neg_integer()}. They are then used in
execute_task/4, which has no return value, or execute_pid/4,
which returns a pid or an inline result. The functions
concurrency_types/0, history/1 and history/3 are used to see
information about the currently configured concurrency limiters.
Merge cxy_ctl