Memory leak (kind of) fix

- objectsKnownToThem is supposed to track if it's necessary to send inv
  commands to a peer
- it is supposed to enter garbage collection after 1 hour
  (ObjectTracker.trackingExpires)
- due to peer not announcing all objects, or announcing them before we
  announce, this contains excessive number of entries after connection
  is fully established.
- profiling revealed that this creates unnecessary memory to be kept
  allocated
- this patch will prevent tracking of entries sent during bigInv,
  reducing the memory "leak"
- it is possible, in theory, that this does have some negative effect,
  like increased bandwidth or neglecting to announce some invs. It
  probably doesn't though as my review of objectsKnownToThem occurrences
  didn't reveal any such case, and since the dict didn't track fully
  accurately anyway (so it would have already been broken if it was a
  problem), I consider it an acceptable risk at the moment. If it indeed
  causes problems, they can be solved separately
- I tested this on one of the bootstrap servers with little memory, and
  it increased the number of connections than can be handled by a factor
  of about 3.5

src/network/tcp.py

@@ -172,7 +172,7 @@ class TCPConnection(BMProto, TLSDispatcher):
                     if Dandelion().hasHash(objHash):
                         continue
                     bigInvList[objHash] = 0
-                    self.objectsNewToThem[objHash] = time.time()
+                    #self.objectsNewToThem[objHash] = time.time()
         objectCount = 0
         payload = b''
         # Now let us start appending all of these hashes together. They will be