blocklib: cache (and used cached) block instances properly in $pageblocks

blocklib: cache (and used cached) block instances properly in $pageblocks

Now blocks_have_content() caches instantiated block objects inside $pageblocks
and blocks_print_blocks() uses them if available. This behaviour now matches
the documentation: blocks instances are created only once, get_content()
may be invoked several times.

A better fix would be to cache the _output_ of the block (the ->content
property) but it may bite us if any block is counting on being called twice.

Discussion at: http://moodle.org/mod/forum/discuss.php?d=45867