Add db.contains handler to portal wire protocol
Currently we just use the dbGet handler to check if we have certain data. As this returns data it is obviously overkill for its task and this can be faster by adding a contains handler to the wire protocol.

See also benchmarks done on a 12GB database:
```
All timings are in ms and are cpu time.
     Average,       StdDev,          Min,          Max,      Samples,         Test 
       0.088,        0.012,        0.067,        0.149,          100, DB put for content
       0.058,        0.014,        0.038,        0.089,          100, DB get for content
       0.005,        0.001,        0.004,        0.008,          100, DB contains for content
       0.127,        0.473,        0.048,        4.804,          100, DB delete for content
       0.018,        0.002,        0.014,        0.031,          100, Get DB size
       0.009,        0.001,        0.008,        0.017,          100, Get DB used size
     136.885,       68.096,      127.954,      810.501,          100, Get DB content size
      19.428,        0.450,       18.993,       22.686,          100, Get DB content count
      97.227,        2.815,       93.321,      112.532,          100, Get DB largest distance
```

So in this case this should result in 10x speedup on the check if the database has the data (and thus in a faster response on accepting or not the offer + better at avoiding overload on offer spams).