Non-Astra databases should require explicit keyspace

[sl-at-ibm]

The default_keyspace default should only apply to Astra databases.
On other destinations (e.g. HCD), spawning a Database instance should always require an explicit keyspace.

ref. slack thread.

[skedwards88]

Changed to "no docs impact" since I am documenting as if this requirement is already in place. (Because documenting that it is only required if you don't already have a keyspace called "default_keyspace" is a weird user experience IMO.)

[sl-at-ibm]

A reasonable approach: condition the default on the destination type, allowing null (for the non-Astra cases).
Then, several methods would do a Guard.NotNull(...) (or a custom version thereof) as their first line, with a message such as "no keyspace set. [etc etc]".