whisper: close the `done` channel in one location

whisper: filereader mode introduced to wnode

cmd/faucet: update state in background, skip when busy

Replace "atleast" with "at least" in the manifest error message.

whisper: topics replaced by bloom filters in mailserver communication

* metrics: add sleep to test in order to get predictable output

* metrics: relax constraints on timer test

The diagnostic tool was saving the unencrypted version of the messages, which is an obvious
security flaw. As of this commit:
  * encrypted messages saved instead of plain text.
  * all messages are stored, even that created by the user of wnode.

whisper: message filtering optimization

Only run the message through filters who registered their interest.

whisper: fixed dataraces in peer unit tests

whisper: mailserver no longer supports the signature validation

Mailserver is provided as an example, but client validation belongs to the upper layer protocol and
needs not be covered in this example. The check that was previously available hinders the switch
to libp2p so we agreed not to include that check in that example code anymore.

internal/ethapi: fix getTransactionReceipt

travis, build, consensus: drop support for Go 1.7

The function would return false for numbers, so isLetter is a more
accurate description of the behavior.

- added a case error struct that contains information about certain error cases
in which we would like to output more information to the client
- added a validation method that iterates and adds the information that is
stored in the error cases

* eth: added travis build status for master branch

* README: fix travis badge order, link to CI

Fixes #3326, #14472

whisper: refactor wnode to systematically store messages if a directory is provided