HeaderMap: C string to string_view conversion follow ups

*Description*:

These are various places where we need to follow up to complete removal of C style strings in the header map.

1. `HeaderString::find(const char* str)` can probably be eliminated entirely by converting call sites to use `getStringView().find()`.
1. `StringUtil::atoull` currently requires creation of some temporary `std::string` objects because it expects C strings, and also returns a C style string. Most call sites ignore the return value, so they can be converted to use `absl::SimpleAtoi` or a new `StringUtil` function that takes and returns string views.
1. `UuidUtils::uuidModBy()` should be migrated to take `absl::string_view`
1. `Span::setOperation()` should be migrated to take `absl::string_view`

*Context*:
https://github.com/envoyproxy/envoy/issues/6494
https://github.com/envoyproxy/envoy/pull/6564

**Action item for CVE-2019-9900**