BlobInputStream should implement skip about pgjdbc

[vlsi]

In fact, the current implementation of reset resets the cached data, so there's no much value in testing mix of read/mark/reset calls.

mark/reset is tested in org.postgresql.test.jdbc2.BlobTest#markResetStream

[davecramer]

Thanks for the suggestion.
Pull requests are welcome.

[OrangeDog]

I don't see any existing tests for the class. LargeObjectManagerTest does a basic read but that seems to be it?
StrangeInputStream implements skip, but nothing ever calls it.

[vlsi]

I guess we do not have specific tests for skip as we used the default implementation.
Sure if we add a custom implementation, we'll need to cover it with tests.

The mix of read + mark + skip + reset might be problematic for testing.

[OrangeDog]

You have implemented mark, reset, and close, but there are apparently no tests for those. I am happy to add this feature, but I don't really want the burden of testing the whole thing from scratch. Unless I simply haven't found the tests?