Ping automatically every 2 minutes (unconditionally)

... instead of after 30 minutes of no sending, for latency measurement
and keep-alive. Also, disconnect if no reply arrives within 20 minutes,
instead of 90 of inactivity (for peers supporting the 'pong' message).
remove unused code from getblockchaininfo()
Merge pull request #4328

f379047 remove unused code from getblockchaininfo() (Philip Kaufmann)
Add testnet DNS seed of Alex Kotenko.
Merge pull request #4330

c30329a Add testnet DNS seed of Alex Kotenko. (Andreas Schildbach)
Merge pull request #2784

f1920e8 Ping automatically every 2 minutes (unconditionally) (Pieter Wuille)
add NetworkIDString() to chainparams

- returns the BIP70 network string
- use that new function in the core and GUI code and remove unused code
  and functions