Decrease LtacProf overhead when not profiling
Decrease LtacProf overhead when not profiling

Note that this slightly changes the semantics of backtracking across
`start ltac profiling`.

SkySkimmer/coq,JasonGross/coq,JasonGross/coq,JasonGross/coq,gares/coq,coq/coq,Zimmi48/coq,silene/coq,coq/coq,gares/coq,gares/coq,JasonGross/coq,Zimmi48/coq,Zimmi48/coq,Zimmi48/coq,ejgallego/coq,coq/coq,SkySkimmer/coq,SkySkimmer/coq,gares/coq,ejgallego/coq,SkySkimmer/coq,silene/coq,silene/coq,coq/coq,gares/coq,ejgallego/coq,ejgallego/coq