ovsdb-data: Expose guts of ovsdb_symbol_table() to clients.

ovsdb-data: Expose guts of ovsdb_symbol_table() to clients.

ovs-vsctl will, in upcoming commits, want to more closely examine its
ovsdb_symbol_table structures.  This could be done by providing a more
complete API, but it doesn't seem worth it to me.  This commit instead goes
the other way, exposing the internals to clients.  This commit also
eliminates the ovsdb_symbol_table_find_uncreated() function, which
ovs-vsctl can now implement itself.