More changes to the extension API (this is only part-way there, but it
builds cleanly so I'm going to commit it).  Change the extension API
handle methods to take the handle as their first argument, get rid of
the WT_EXTENSION_API.default_session method, instead, store a reference
to the enclosing WT_CONNECTION in the extension handle.

This push also includes a few changes that are part of creating a STEC
data source.
Replace wiredtiger_extension_api() with WT_CONNECTION.get_extension_api.

Pass the wiredtiger_extension_init function a (WT_CONNECTION *) instead
of a (WT_SESSION *, WT_EXTENSION_API) pair.
Change the extension entry point signature to take a (void *config)
instead of a const char *config, and pass in a cfg[], that way the
entry point function can use WT_EXTENSION_API::config to parse the
configuration.
Rename WT_EXTENSION_API::config to be WT_EXTENSION_API::get_config.
Replace WT_EXTENSION_CONFIG.argv[] with WT_EXTENSION_API::get_config_next;
that method returns the next entry from the list of items contained in the
last value retrieved from WT_EXTENSION_API::get_config.