Refactor: liblrmd: functionize key cache better

This separates read_gnutls_key(), which previously retrieved the
Pacemaker Remote key from either cache or a file, into multiple functions.

read_gnutls_key() now only reads a key from a file.

The key cache has been made into a struct with accessor functions, for
code isolation and readability, and to make future changes easier.

get_remote_key() is a new function that retrieves the key from cache or file.