drreg should not lazily restore a reg in a DR TLS slot across an app instr

[derekbruening]

drreg uses DR's TLS slots once it runs out of the N slots that the client
requested in drreg_init(). However, while the drreg docs mention that DR
slots are not safe across app instrs, drreg itself goes ahead and does its
lazy restore optimization across app instrs regardless of whether in a DR slot.

A separate question is whether drreg should let a client hold a
reservation across an app instr if it's in a DR slot?