@Override
    public void mark(int readlimit) {
        in.mark(readlimit);
        mark = firstOffsetInLastRead;
        // it's okay to mark even if mark isn't supported, as reset won't work
    }
Creates a <code>FilterInputStream</code>
by assigning the  argument <code>in</code>
to the field <code>this.in</code> so as
to remember it for later use.

@param in the underlying input stream, or <code>null</code> if
          this instance is to be created without an underlying stream.
stroom-data/stroom-data-store-impl/src/main/java/stroom/data/store/impl/LocationAwareInputStream.java