Support index-only scans for type DOUBLE

[spetrunia]

(branching this off from issue #26)

Currently, index-only scans are not supported for column type DOUBLE.

Testcase:

create table t31 (pk int auto_increment primary key, key1 double, key(key1)) engine=rocksdb;
insert into t31 values (),(),(),(),(),(),(),();
explain select key1 from t31 where key1=1.234;

It is actually possible to restore double from its mem-comparable form and thus support index-only scans.
See filesort.cc: void change_double_for_sort(double nr,uchar *to) for the code that needs to be inverted.