Add a URI-specific version of wt_conn_btree_apply.

@keithbostic and @michaelcahill please review this.  This fixes the specific issue in #1439.

This cuts the time in my test from 84 seconds to about 4.5 seconds for 3000 tables.  I created a new, slightly different version,` __wt_conn_btree_apply_uri` that applies the function to any dhandle that matches the URI.  This avoids walking the entire `conn->dhandle` list to then walk the session list to then call the function on all URIs and have it be a no-op since the URI does not match the one we're interested in.