Cache the TlsConnector built from SslOpts

[goffrie]

Constructing a TlsConnector is fairly expensive - it can involve searching for and parsing certificates, for example.
Both native-tls and rustls therefore allow a single TlsConnector to be used to connect many times.

To avoid adding a fallible (and async) initialization step and therefore preserve the public API of this crate, this PR caches the TlsConnector beside the SslOpts using a tokio OnceCell, so that any error from its construction still gets emitted at the same point.

The SslOpts and corresponding TlsConnector are stored together in a struct in order to avoid accidentally mutating the SslOpts without clearing the connector.

I've also slightly refactored the no-TLS case to behave more uniformly with the rustls and native-tls implementations.

[harsh-98]

We are still calling clone on the TlsConnector that we get from native tls or rustls. https://github.com/blackbeam/mysql_async/blob/master/src/opts/mod.rs#L1179.

[goffrie]

Correct. In all implementations (rustls, openssl, securityframework, schannel), the underlying configuration object is reference counted and cheaply cloneable.