Minimize external header dependencies

[jagerman]

Currently the libquic public headers directly include ngtcp2, libevent, and oxenc, which means any application building against libquic must necessarily also build against the upstream dependencies. It ought to be possible to eliminate those, though, to make them private dependencies of libquic (i.e. to only include them from inside src/, but not include/.).

Just to take one example: quic/crypto.hpp currently includes ngtcp2 headers, but only appears to need them for the conn_ref member, which could instead be implemented as a struct ngtcp2_crypto_conn_ref; predeclaration and changing conn_ref to a unique_ptr, and adding an explicit destructor in the header with TLSSession::~TLSSession() = default; in the .cpp.

Obviously this is just one case of many, but it would be nice to privatize the dependencies.