[autocomplete] Merge SQL function library with the output from show functions

Impala has multiple:

show functions;
show aggregate functions;
show analytic functions;
show functions in _impala_builtins;
show aggregate functions in _impala_builtins;
show analytic functions in _impala_builtins;

And we should include the active database.

SELECT | FROM tbl; – built in + active db
SELECT db.| FROM tbl; – built-in + functions in db