Increase maximum sharing map tree height from 6 to 10

Now that the sharing map tree grows dynamically as elements are inserted, a
higher value for the maximum tree height can be used. The maximum tree height is
determined by bits / chunk, with bits being the number of bits of the hash code
used, and chunk being the number of bits used to determine the child node of an
inner node. For example, if chunk = 3, then a node can have at most 8 children.
Then if bits = 30, bits / chunk = 10 and the maximum tree height is 10. The
constant bits should be divisible by chunk.

Author: danpoe

src/util/sharing_map.h

@@ -1284,7 +1284,7 @@ SHARING_MAPT2(optionalt<std::reference_wrapper<const, mapped_type>>)::find(
 
 SHARING_MAPT(const std::string)::not_found_msg="key not found";
 
-SHARING_MAPT(const std::size_t)::bits = 18;
+SHARING_MAPT(const std::size_t)::bits = 30;
 SHARING_MAPT(const std::size_t)::chunk = 3;
 
 SHARING_MAPT(const std::size_t)::mask = 0xffff >> (16 - chunk);