discovery: slowly increase sampling size

Some pathological discovery runs can requires many roundtrip. When this happens

things can get very slow.

To make the algorithm more resilience again such pathological case. We slowly

increase the sample size with each roundtrip (+5%). This will have a negligible

impact on "normal" discovery with few roundtrips, but a large positive impact of

case with many roundtrips. Asking more question per roundtrip helps to reduce

the undecided set faster. Instead of reducing the undecided set a linear speed

(in the worst case), we reduce it as a guaranteed (small) exponential rate. The

data below show this slow ramp up in sample size:

round trip | 1 | 5 | 10 | 20 | 50 | 100 | 130 |

sample size | 200 | 254 | 321 | 517 | 2 199 | 25 123 | 108 549 |

covered nodes | 200 | 1 357 | 2 821 | 7 031 | 42 658 | 524 530 | 2 276 755 |

To be a bit more concrete, lets take a very pathological case as an example. We

are doing discovery from a copy of Mozilla-try to a more recent version of

mozilla-unified. Mozilla-unified heads are unknown to the mozilla-try repo and

there are over 1 million "missing" changesets. (the discovery is "local" to

avoid network interference)

Without this change, the discovery:

- last 1858 seconds (31 minutes),
- does 1700 round trip,
- asking about 340 000 nodes.

With this change, the discovery:

- last 218 seconds (3 minutes, 38 seconds a -88% improvement),
- does 94 round trip (-94%),
- asking about 344 211 nodes (+1%).

Of course, this is an extreme case (and 3 minutes is still slow). However this

give a good example of how this sample size increase act as a safety net

catching any bad situations.

We could image a steeper increase than 5%. For example 10% would give the

following number:

round trip | 1 | 5 | 10 | 20 | 50 | 75 | 100 |

sample size | 200 | 321 | 514 | 1 326 | 23 060 | 249 812 | 2 706 594 |

covered nodes | 200 | 1 541 | 3 690 | 12 671 | 251 871 | 2 746 254 | 29 770 966 |

In parallel, it is useful to understand these pathological cases and improve

them. However the current change provides a general purpose safety net to smooth

the impact of pathological cases.

To avoid issue with older http server, the increase in sample size only occurs

if the protocol has not limit on command argument size.