Change default maxCachedBinarySize to match lucene DEFAULT_BLOB_SIZE

The default for DataStoreBlobStore#maxCachedBinarySize was supposed to be equal or greater than OakDirectory#DEFAULT_BLOB_SIZE. That default was increased to 32 * 1024 while here the default is still set to 17 * 1024. Also, there the newer lucene files uses IndexDefinition#DEFAULT_BLOB_SIZE which is 1024*1024 -1024.
We should increase the default to 1024 * 1024 i.e. 1MB