Cert loading from PEM & restructuring 

- Added cert loading

- Extracted X509 tests

- Extracted `hash_str_to_vec` function into `utils.rs` as it is used 
  in a couple of tests now

- Fixed `hash_str_to_vec`, funny enough prev test with it was working 
  because I’ve made another mistake by providing VerifyNone instead of 
  VerifyPeer and those 2 mistakes compensated each other. Since
  `hash_str_to_vec` is private there was no errors on client code
Better thread working

Works pretty stable for me, although I'm not sure it is 100% thread safe yet.

Changes mostly inspired by https://www.arangodb.org/2014/07/08/started-hate-openssl :

- Switched to newer CRYPTO_THREADID API for Unixes

- Clearing errors queue before every critical operation
Simplified SslStream.write, updated comments
Locking internal buffer from same thread access

If cloned stream was used in the same thread
WIP: erase later