Gets the set of active producers. @brief Gets the set of active producers.
@param producers - Pointer to a buffer of account names @param datalen - Byte length of buffer, when passed 0 will return the size required to store full output.
@return uint32_t - Number of bytes actually populated
producers is a pointer to a range of memory at least
datalen bytes long
@post the passed in
producers pointer gets the array of active producers.
@code capi_name producers; uint32_t bytes_populated = get_active_producers(producers, sizeof(capi_name)*21); @endcode