This is only ever called with a recursive space map wrapper,
and that causes too many false positives. False positives trigger
a copy of a metadata page, so this is a false economy.
The compiler is unable to create a default desctructor for the derived
class missing_counts if the virtual destructor for the class damage is
private.
This fixes compilation bugs with CXXFLAGS=-std=gnu++11 together with
gcc 4.8.3 and boost 1.55.
new_block() is now a concrete method implemented using the virtual
find_free() and inc() methods.
recursive space map is better at giving correct reference counts.
