This patch makes a minor optimization to the restrpos() and markpos()
implementations for btree and hash: to bump the reference count on a
buffer, it is more efficient to use IncrBufferRefCount() than to do a
ReadBuffer (for one thing we needn't acquire the BufMgrLock; we also
don't need to do a hash table lookup etc.)
Barring any objections I will apply to HEAD before end of day.
-Neil
