After checkptr fixes by 2fc6815, it was discovered that new issues
were hit in production systems, in particular when a single process
opened and updated multiple separate databases.  This indicates that
some bug relating to bad unsafe usage was introduced during this
commit.

This commit combines several attempts at fixing this new issue.  For
example, slices are once again created by slicing an array of "max
allocation" elements, but this time with the cap set to the intended
length.  This operation is espressly permitted according to the Go
wiki, so it should be preferred to type converting a
reflect.SliceHeader.