Fix hard to find crash in MLDelayableTimer

This crash seems to be a bug in a compiler optimization, releasing the
MLDelayableTimer object and its ivars (like the _cancelHamdler block)
and then trying to call the _cancelHandler ivar (which is nil at this point).

I had to disassemble the Mach-O binary in our Monal.ipa release file
using llvm-otool and use that disassembly together with the crash report
to reconstruct what was happening.

000000000055057c	mov	x0, x19
0000000000550580	bl	_objc_msgSend$invalidate
0000000000550584	mov	x0, x19
0000000000550588	bl	0x564908 ; symbol stub for: _objc_sync_exit
000000000055058c	mov	x0, x19
0000000000550590	bl	0x56486c ; symbol stub for: _objc_release	; <-- unexpected ARC release of self in [MLDelayableTimer cancel]
0000000000550594	ldr	x0, [x19, #0x10]                                ; <-- load of now zeroed _cancelHandler ivar that should point to an objc block
0000000000550598	ldr	x2, [x0, #0x10]                                 ; <-- crash while trying to access the pointer to the code inside our objc block (we try to access the invalid address 0x0000000000000010)
000000000055059c	mov	x1, x19
00000000005505a0	ldp	x29, x30, [sp, #0x30]
00000000005505a4	ldp	x20, x19, [sp, #0x20]
00000000005505a8	add	sp, sp, #0x40
00000000005505ac	br	x2

Monal/Classes/MLDelayableTimer.m

@@ -81,7 +81,7 @@ -(void) pause
             return;
         }
         NSTimeInterval remaining = _wrappedTimer.fireDate.timeIntervalSinceNow;
-        if(remaining == 0)
+        if(remaining < 0.001)
         {
             DDLogWarn(@"Tried to pause timer the exact second its firing: %@", self);
             return;
@@ -121,8 +121,11 @@ -(void) cancel
         }
         DDLogDebug(@"Canceling timer: %@", self);
         [self invalidate];
+        [self scheduleBlockInRunLoop:^{
+            if(self->_cancelHandler != nil)
+                self->_cancelHandler(self);
+        }];
     }
-    _cancelHandler(self);
 }
 
 -(void) invalidate