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.
Merge pull request #567 from wiredtiger/metadata-move

Have data sources manage their own metadata.