reactor: move optional to experimental so we compile with c++14
reactor: move optional to experimental so we compile with c++14

Using dialect c++14, seastar apps won't compile. The reason is that
std::optional is not recognized in c++14 - only
std::experimental::optional is.

To make a compabitibily effort, let's keep the experimental, which
works well on both c++14 and c++17.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>
Message-Id: <20180430162935.9741-1-7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

syuu1228/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,scylladb/seastar,dreamsxin/seastar,dreamsxin/seastar,cloudius-systems/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar