Use modifyMVar in usingState for exception safety
Use modifyMVar in usingState for exception safety

Also, force the state value, to avoid potential space leaks.

lancelotsix/hs-tls,AaronFriel/hs-tls,beni55/hs-tls,beni55/hs-tls,tolysz/hs-tls,AaronFriel/hs-tls,erikd/hs-tls,tolysz/hs-tls,erikd/hs-tls,lancelotsix/hs-tls