msg/async: avoid referencing the temporary string

msg/async: avoid referencing the temporary string

* get_val<std::string>(...) returns a temporary std::string, so we cannot
  keep a reference to it after evaluating this method. so convert it to
  an integer right away in the same expression.
* use std::stoull() with base = 16, so we can parse a hex string
  representing up to 64 bits.

Signed-off-by: Kefu Chai <kchai@redhat.com>