This change refactors and hardens parts of data node management
functionality.
* A number of of permissions checks have been added to data node
management functions. This includes checking that the user has
proper permissions for both table and server objects.
* Permissions checks are now done when creating remote chunks on data
nodes.
* The add_data_node() API function has been simplified and now returns
more intuitive status about created objects (foreign server,
database, extension). It is no longer necessary to specify a user to
connect with as this is always assumed to be the current user. The
bootstrap user can still be specified explicitly, however, as that
user might require elevated permissions on the remote node to
bootstrap.
* Functions that capture exceptions without re-throwing, such as
`ping_data_node()` and `get_user_mapping()`, have been refactored to
not do this as the transaction state and memory contexts are not in
states where it is safe to proceed as normal.
* Data node management functions now consistently check that any
foreign servers operated on are actually TimescaleDB server objects.
* Tests now run with a superuser a regular user specific to
clustering. These users have password auth enabled in `pg_hba.conf`,
which is required by the connection library when connecting as a
non-superuser. Tests have been refactored to bootstrap data nodes
using these user roles.
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