add 4.10 docvaluesformat

We can improve the current format in a few ways:

• speed up Sorted/SortedSet byte[] lookup by structuring the term blocks differently (allow random access, more efficient bulk i/o)
• speed up reverse lookup by adding a reverse index (small: just every 1024'th term with useless suffixes removed).
• use slice API for access to access to binary content, too.