Expose private m_version for table

[nhachicha]

Fix realm/realm-java#1676
we need to expose m_version (in table.hpp) in order to implement a more fine-grained notification & avoid calling RealmResults (table_view) & RealmObject (Row) callbacks for every commit.

comparing the old & current value of m_version allow us to know if the new commit impacted our table_view/row, this should reduce the number of invocations to callbacks.

as discussed here #1254

[finnschiermer]

We need some freedom for implementations in core. We can provide a guarantee that if the version of a table is unchanged, so is the table. The opposite is not true: the version may change even if the table is unchanged. Also, we define the table as being only the table, giving no guarantees as to changes to link targets.

[finnschiermer]

Available as PR #1357