Update implementation status
Default to frozen initial dataset options hash

This is a performance optimization.  If the initial dataset
options hash is frozen, then all initial datasets can share
a frozen hash instance.
Remove Dataset#query from informix adapter

This conflicts with the query extension, and is different from all
of the other adapters.
Rename dataset_set_overrides to set_overrides

Update implementation status while here.
Reword reciprocal condition/block issue

If the current association has conditions or a block, but the
reciprocal association does not, it should be safe to use
the reciprocal.  Only if the reciprocal association has a
condition or block should it not be safe. Example:

  Album.many_to_one :artist
  Artist.one_to_many :a_albums, :conditions=>{:name=>/^a/i},
    :class=>Album

Here, artist should not pick a_albums as a reciprocal, since the
album name may not start with "a".  However, a_albums should have
artist as the reciprocal, since any matching album should have
that artist.
Update implementation status
Remove sequel_3_constants extension

It's very difficult to add deprecation messages for possible
constant use.  I expect Sequel::Error::Rollback is still common
enough that removing it would break a significant amount of
code.  So The Sequel::Error::* constants will stay.  The
Sequel::SQL::SQLArray and Sequel::SQL::NoBooleanInputMethods
constants can go away, as it is unlikely anyone is using them
directly.

Sequel::NotImplemented I'm deprecating completely.  Any code
that rescues that extension will break in Sequel 4, so all places
in Sequel that raise that extension now have deprecation warnings.