Prior to this change, the ShallowEtagHeaderFilter would use a
ResizableByteArrayOutputStream to internally write data and calculate
the ETag. While that implementation is faster than the regular
ByteArrayOutputStream (since it has a better strategy for growing the
internal buffer), a lot of buffer copying/writing still happens.
This change adds a new FastByteArrayOutputStream implementation that
internally uses a LinkedList<Byte[]> to store the content. So when
writing bytes to that OutputStream implementation, new byte[] are
added to the list when the previous ones are full. This saves most
of the instantiating/copying operations.
Note that new methods were added in DigestUtils to allow usage of
Streams instead of byte[], which is more efficient in our case.
Fixes#653
Issue: SPR-12081
213a3fd779