* Improve comparison and printing of Version
There are two members in `Version`:` first` and `second`.
There are many places where we need to print one `Version` object and
compare two `Version` objects, but in the current code, these two members
are accessed directly, which makes the code very tedious.
This patch mainly do:
1. Adds overloaded methods for `operator<<()` for `Version`, so
we can directly print a Version object;
2. Adds the `cantains()` method to determine whether it is an containment
relationship;
3. Uses `operator==()` to determine if two `Version` objects are equal.
Because there are too many places need to be modified, there are still some
naked codes left, which will be modified later.
This patch also removes some necessary header file references.
No functional changes in this patch.
Remove the default constructor for UniqueID
Add a gen_uid method in UniqueId. If need to generate a new uid, users should call this api explicitly.
Reuse boost random generator not generate a new one every time.
NOTE: This patch would modify all Backend's data.
And this will cause a very long time to restart be.
So if you want to interferer your product environment,
you should upgrade backend one by one.
1. Refactoring be is to clarify the structure the codes.
2. Use unique id to indicate a rowset.
Nameing rowset with tablet_id and version will lead to
many conflicts among compaction, clone, restore.
3. Extract an rowset interface to encapsulate rowsets
with different format.
