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().
2. 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.
3. UuidUtils::uuidModBy() should be migrated to take absl::string_view
4. 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