hdf5

[svn-r23473] Update the VOL plugin callbacks with better async I/O support:
 - change hid_t req in callbacks to void **req
 - change all callbacks to accept dxpl_ids (except dataset write and read which already do)
 - update all the private and public VL routines to accept a dxpl_id and event queue id
 - Add an async class for the VOL plugin with cancel, test, and wait request operations
 - add the external VOL log plugin test to the examples directory

NOTE that async operations are still not provided by the HDF5 API, so
all event queue IDs and requests passed to all plugins are NULL for
now.

Tested on jam.