Late joiners plus view changes

[olgavrou]

Resolves #837
Resolves #859

• if a request has been executed on view v for a majority of replicas but but is executed during the view change from v to v' on a different replica, then the request is seen as executed on view v. This means it will be encrypted with view v set in the IV and the pre-prepare will be stored in the ledger with view v for this replica
• various small bug fixes
• global commit is called on commit and not on prepare (PBFT should globally commit on commit not on prepare #859) except for when f == 0 where we need to globally commit earlier
• remove test_UDP_with_delays and related files

late joiners test changed slightly to avoid adding nodes after the network has been opened as we don't currently support reconfiguration with pbft. It now starts with 3 active replicas, and the late joiner is the fourth replica

[ghost]

view_changes@6885 aka 20200401.3 vs master ewma over 50 builds from 6077 to 6882

[ghost]

view_changes@6765 aka 20200327.83 vs master ewma over 30 builds from 6191 to 6754