Check tables and views against current_schemas

Currently, tables and views return tables and views in all schemas
except those specifically excluded.  However, that leads to
situations where they return tables that can't be referenced
unqualified.
Check client_min_messages on PostgreSQL

This setting currently uses insecure quoting, leading to the
possibility of SQL injection.  Since there are only a few
allowable settings, checking the input against the valid ones
will be sufficient to prevent SQL injection without undue
restriction.
Make defaults_setter plugin lazy

Currently, upon initialize, defaults_setter sets the value of all
of the keys, which causes a significant slowdown.  An alternative
approach is just to override the accessor methods, and use the
default value if there is no entry in values.  This approach should
be better for performance, but is not completely backwards
compatible, as looking directly in the values would not reflect
the defaults.
Remove k_require, ts_require, tsk_require

Way back when, ruby's require wasn't thread safe.  However, I think
all ruby versions in current use are (even 1.8.7).  There is no
need for this extra complexity anymore.
Move Sequel.empty_array_handle_nulls to extension

This is currently only used in a single place, and doesn't
warrant a global setting now that Database.extension can
get an extension to work the same way.
Remove Sequel::Dataset.introspect_all_columns

There is no need for special setting for global behavior, now that
Sequel::Database.extension :columns_introspection can be used.
Remove global changes from some extensions

These extensions all modify Database/Dataset specific behavior,
so they really should be switched to Database/Dataset extensions
instead of global extensions.  You'll still be able to get the
global behavior using Sequel::Database.extension.