Two bugfixes regarding stale executable files, and executables changing between restarts (#689)

* service/debugger: Restore breakpoints using file:line on restart

Restoring by address can cause the breakpoint to be inserted in the
middle of an instruction if the executable file has changed.

* terminal: Warn of stale executable when printing source

service/client.go

@@ -1,6 +1,8 @@
 package service
 
 import (
+	"time"
+
 	"github.com/derekparker/delve/service/api"
 )
 
@@ -10,11 +12,14 @@ type Client interface {
 	// Returns the pid of the process we are debugging.
 	ProcessPid() int
 
+	// LastModified returns the time that the process' executable was modified.
+	LastModified() time.Time
+
 	// Detach detaches the debugger, optionally killing the process.
 	Detach(killProcess bool) error
 
 	// Restarts program.
-	Restart() error
+	Restart() ([]api.DiscardedBreakpoint, error)
 
 	// GetState returns the current debugger state.
 	GetState() (*api.DebuggerState, error)