rgw,mon,common/strtol: use strict_iec_cast(std::string_view, ..)

rgw,mon,common/strtol: use strict_iec_cast(std::string_view, ..)

this variant is better than strict_iec_cast(const char*), because:

* we can just pass std::string to it, as std::string_view can be
  constructed from a std::string implicitly
* strict_iec_cast(std::string_view, ..) is the underlying
  implementation of strict_iec_cast(const char*,..), so less
  indirection helps with the readability.
* rgw,mon: use strict_iec_cast(std::string_view, ..) instead.

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