Creating an easy handle polls file system every time for OpenSSL

[sagebind]

While doing some profiling of Isahc to see if I could squeeze any more performance out of it, I noticed that creating new easy handles was spending a noticeable amount of time in openssl_probe::probe. Not an egregious amount of time, mind you, but enough to be noticeable. Digging deeper I saw that this function allocates a fair number of PathBufs and makes std::fs::metadata calls to look for certificate files.

It is probably unlikely for these files to be changed at runtime, and if they do, I don't think most people would expect a running program to immediately pick up the change. Therefore I suggest that we lazily probe for certs just once, and re-use the results for subsequent easy handles created. Or at the very least, allow the user to opt-out of scanning for certs for every easy handle created.

[alexcrichton]

Oh dear I think that function is intended to only be called once, it definitely shouldn't need to be called each time!