extract callback handling into separate function
extract callback handling into separate function

This opens up the possibility to override the callback handling in one
central place and could be used to provide an option to enable a
"legacy" callback style where the first argument (i.e. `err`) is always
a valid status object.

aerospike/aerospike-client-nodejs,aerospike/aerospike-client-nodejs,aerospike/aerospike-client-nodejs,aerospike/aerospike-client-nodejs,aerospike/aerospike-client-nodejs