This adds a new class Net::SSH::KnownHosts::Cached which loads the
known_hosts files once, builds an index, and uses that when searching
for keys for a given host. An instance of this object can be passed
using the `known_hosts` option in order to avoid loading known_hosts
multiple times when making multiple connections to either the same host
or several hosts.

This behavior is implemented in a new class because it differs from the
existing implementation in the case where the content of the known_hosts
file changes outside the current process. With the standard
implementation, those changes will always be picked up the next time a
connection is made, whereas the Cached implementation won't
automatically incorporated changes made out-of-band.

When the cache is built, store the file modification times. Only re-build the cache when the mtime does not exist or has changed since last read. Similarly with the add method, only rebuild the cache when the write is the only change since the last read.

Instead of tracking mtime to determine if known_hosts need to be re-parsed, track file sha256 checksums. This avoids cases where mtimes are close enough together that a change goes un-noticed (discovered writing tests). Also based on feedback also invalidate cache if known_hosts file is deleted OOB.

This commit ensures file paths are expanded given the defaults contain `~`. It also handles the case where *no* known host files are readable by intializsing caches to empty hashes in the init method.