Inefficiency in SNI policy parsing

It seems that rather than parsing these types of "text" configs once, we parse it over and over again (strcmp()) on each transaction hitting this code:

```
std::string_view
HttpSM::get_outbound_sni() const
{
using namespace ts::literals;
ts::TextView zret;
ts::TextView policy{t_state.txn_conf->ssl_client_sni_policy, ts::TextView::npos};

if (policy.empty() || !strcmp(policy, "host"_tv)) {
// By default the host header field value is used for the SNI.
int len;
char const *ptr = t_state.hdr_info.server_request.host_get(&len);
zret.assign(ptr, len);
} else if (ua_txn && !strcmp(policy, "server_name"_tv)) {
zret.assign(ua_txn->get_netvc()->get_server_name(), ts::TextView::npos);
} else if (policy.front() == '@') { // guaranteed non-empty from previous clause
zret = policy.remove_prefix(1);
} else {
// If other is specified, like "remap" and "verify_with_name_source", the remapped origin name is used for the SNI value
zret.assign(t_state.server_info.name, ts::TextView::npos);
}
return zret;
}
````