Merge #12134: Build previous releases and run functional tests

c456145b2c65f580683df03bf10cd39000cf24d5 [test] add 0.19 backwards compatibility tests (Sjors Provoost)
b769cd142deda74fe46e231cc7b687a86514f2f1 [test] add v0.17.1 wallet upgrade test (Sjors Provoost)
9d9390dab716f07057c94e8e21f3c7dd06192f35 [tests] add wallet backwards compatility tests (Sjors Provoost)
c7ca6308968b29a0e0edc485cd06e68e5edb7c7d [scripts] support release candidates of earlier releases (Sjors Provoost)
8b1460dbd1b732f06d4cebe1fa6844286c7a0056 [tests] check v0.17.1 and v0.18.1 backwards compatibility (Sjors Provoost)
ae379cf7d12943fc192d58176673bcfe7d53da53 [scripts] build earlier releases (Sjors Provoost)

Pull request description:

  This PR adds binaries for 0.17, 0.18 and 0.19 to Travis and runs a basic block propagation test.

  Includes test for upgrading v0.17.1 wallets and opening master wallets with older versions.

  Usage:

  ```sh
  contrib/devtools/previous_release.sh -f -b v0.19.0.1 v0.18.1 v0.17.1
  test/functional/backwards_compatibility.py
  ```

  Travis caches these earlier releases, so it should be able to run these tests with little performance impact.

  Additional scenarios where it might be useful to run tests against earlier releases:

  * creating a wallet with #11403's segwit implementation, copying it to an older node and making sure the user didn't lose any funds (although this PR doesn't support `v0.15.1`)
  * future consensus changes
  * P2P changes (e.g. to make sure we don't accidentally ban old nodes)

ACKs for top commit:
  MarcoFalke:
    ACK c456145b2c65f580683df03bf10cd39000cf24d5 🔨

Tree-SHA512: 360bd870603f95b14dc0cd629532cc147344f632b808617c18e1b585dfb1f082b401e5d493a48196b719e0aeaee533ae0a773dfc9f217f704aae898576c19232

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