The server fills radix allocators for the client to consume while
allocating during a transaction. The radix merge function used to move
an entire radix block at a time. With larger blocks this becomes much
too coarse and can move way too much in one call.
This moves allocator bits a word at a time and more precisely moves the
amount that the caller asked for.
Signed-off-by: Zach Brown <zab@versity.com>