Interned strings unification for TS/NTS

Interned strings unification for TS/NTS

Hereby, interned strings are supported in thread safe PHP. The patch
implements two types of interned strings

- interning per process, strings are not freed till process end
- interning per request, strings are freed at request end

There is no runtime interning.

With Opcache, all the permanent iterned strings are copied into SHM on
startup, additional copying into SHM might happen on demand.