Null out closed handles in Java, and detect nulled handles on use.  refs #485.

As reported in #485, when a conn/session is closed, any subordinate
open cursors and sessions have dangling handles that refer to freed
memory.  But also, if a conn/session/cursor handle is closed, that
handle may still be referenced in Java though it references freed
memory.

This commit uses the event handler for close calls to capture the first
case, and makes explicit calls at the close site for the second case.
For each case, a close will set the Java side 'swigCPtr' for each associated
object being closed to 0.

For detection, typemaps effectively check an arg's swigCPtr to see
if the object has been previously closed.  Special cases, like when
a NULL handle is permitted in the API, are accounted for.

Some details:
  - WT_HANDLE_NULLABLE() and WT_HANDLE_CLOSED() mark points in the API
    (wiredtiger.in) where handles may be used as null and where
    handles are closed.  This are not exposed to doxygen.
  - lang_private fields have been added to WT_CONNECTION_IMPL,
    WT_SESSION_IMPL and WT_CURSOR to keep a handle to the
    associated java object, needed at the time of the close().