Add a new extension function __wt_ext_config_strget that returns the
configuration value from a single string instead of an array.

Change memrata code to use the new interface.
The config-scan docs were mixed up, fix them.
lint
Memory leak, found by inspetion -- free the returned value from
metadata_read.
whitespace
Make data-sources responsible for their own metadata entries.
Add standard metadata functions to the extension API.
Rename metadata_read to be metadata_search to follow the other
operation naming (insert, update, remove).
update
lint
Add examples to the WiredTiger extension methods for accessing the
metadata.
Have hot backup retry on EBUSY in test/format.

Fixes issue #566
Merge pull request #567 from wiredtiger/metadata-move

Have data sources manage their own metadata.
Make the "source" key to WT_SESSION::create undocumented.  There are problems associated with using it from application (e.g., rename becomes ambiguous), and we don't have a real world use case yet.