hdf5_der

[svn-r23828] Fix 2 bugs for SWMR access:
1) H5O_load() in H5Ocache.c: when reading a block that is > spec_read siez, read the whole block in again and possibly decode the header.
2) H5F_accum_write() in H5Faccum.c: for a large write that is >= H5F_ACCUM_MAX_SIZE, flush the metadata in the accumulator first before the write.
Tests are added to test/ohdr.c and test/accum.c. h5committested.