Allow CREATE EXTENSION to follow extension update paths.

Previously, to update an extension you had to produce both a version-update
script and a new base installation script.  It's become more and more
obvious that that's tedious, duplicative, and error-prone.  This patch
attempts to improve matters by allowing the new base installation script
to be omitted.  CREATE EXTENSION will install a requested version if it
can find a base script and a chain of update scripts that will get there.
As in the existing update logic, shorter chains are preferred if there's
more than one possibility, with an arbitrary tie-break rule for chains
of equal length.

Also adjust the pg_available_extension_versions view to show such versions
as installable.

While at it, refactor the code so that CASCADE processing works for
extensions requested during ApplyExtensionUpdates().  Without this,
addition of a new requirement in an updated extension would require
creating a new base script, even if there was no other reason to do that.
(It would be easy at this point to add a CASCADE option to ALTER EXTENSION
UPDATE, to allow the same thing to happen during a manually-commanded
version update, but I have not done that here.)

Tom Lane, reviewed by Andres Freund

Discussion: <20160905005919.jz2m2yh3und2dsuy@alap3.anarazel.de>

doc/src/sgml/extend.sgml

@@ -885,6 +885,47 @@ SELECT * FROM pg_extension_update_paths('<replaceable>extension_name</>');
     </para>
    </sect2>
 
+   <sect2>
+    <title>Installing Extensions using Update Scripts</title>
+
+    <para>
+     An extension that has been around for awhile will probably exist in
+     several versions, for which the author will need to write update scripts.
+     For example, if you have released a <literal>foo</> extension in
+     versions <literal>1.0</>, <literal>1.1</>, and <literal>1.2</>, there
+     should be update scripts <filename>foo--1.0--1.1.sql</>
+     and <filename>foo--1.1--1.2.sql</>.
+     Before <productname>PostgreSQL</> 10, it was necessary to also create
+     new script files <filename>foo--1.1.sql</> and <filename>foo--1.2.sql</>
+     that directly build the newer extension versions, or else the newer
+     versions could not be installed directly, only by
+     installing <literal>1.0</> and then updating.  That was tedious and
+     duplicative, but now it's unnecessary, because <command>CREATE
+     EXTENSION</> can follow update chains automatically.
+     For example, if only the script
+     files <filename>foo--1.0.sql</>, <filename>foo--1.0--1.1.sql</>,
+     and <filename>foo--1.1--1.2.sql</> are available then a request to
+     install version <literal>1.2</> is honored by running those three
+     scripts in sequence.  The processing is the same as if you'd first
+     installed <literal>1.0</> and then updated to <literal>1.2</>.
+     (As with <command>ALTER EXTENSION UPDATE</>, if multiple pathways are
+     available then the shortest is preferred.)  Arranging an extension's
+     script files in this style can reduce the amount of maintenance effort
+     needed to produce small updates.
+    </para>
+
+    <para>
+     If you use secondary (version-specific) control files with an extension
+     maintained in this style, keep in mind that each version needs a control
+     file even if it has no stand-alone installation script, as that control
+     file will determine how the implicit update to that version is performed.
+     For example, if <filename>foo--1.0.control</> specifies <literal>requires
+     = 'bar'</> but <literal>foo</>'s other control files do not, the
+     extension's dependency on <literal>bar</> will be dropped when updating
+     from <literal>1.0</> to another version.
+    </para>
+   </sect2>
+
    <sect2 id="extend-extensions-example">
     <title>Extension Example</title>