string: optimize data snapshotting for sso

Until now, data in sso was accessed using pmem::obj::array::operator[].
This operator snapshots data which has some performance cost (even
if the data is already snapshotted). This patch change all calls
to pmem::obj::array::operator[] which are alredy preceded by snapshotting
entire sso, by calls to operator[] on direct array of element (_data member).