Use luaossl ssl object cache

[daurnimator]

Introduced in wahern/luaossl@bbe0e35 and modified in wahern/luaossl@c8c895a

This would allow calling :checktls() multiple times to return the same object. Additionally, it would be the same object as passed to luaossl callbacks (such as setAlpnSelect)

• luaossl could expose the cache somewhere (e.g. require "openssl.ssl".ssl_cache)
• luaossl could expose ssl_push as a C symbol
• luaossl could expose a new (unsafe) ssl pushing function that takes a lightuserdata