TLS support

[jhecking]

Support for Transport Layer Security for all communication between the client and all cluster nodes. Support for all 3 TLS security modes:

• Standard Authentication
• Mutual Authentication
• Encryption-Only

Note: Requires Aerospike Enterprise Server Version 3.11 or later.

[jhecking]

One of the key decisions to make in order to support this is what SSL/TLS library to use in implementing. There exist at least a few different options today:

• openssl
• native-tls
• rustls

Care needs to be taken to avoid dependency issues when depending on -sys crates, e.g. openssl-sys. Ref. hyperium/hyper#985 for some interesting discussion around these issues.

TODO: Do more research to find out what exactly it means to do an implementation that's "generic over SslClient".