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

* core: make current*Block atomic, and accessor functions mutex-free

* core: fix review concerns

* core: fix error in atomic assignment

* core/light: implement atomic getter/setter for headerchain

* Fixed broken link to JavaScript-Console wiki in cmd line help

* cmd/geth: Added missing r in 'JavaScript'

swarm: creates Swarm landing page for browser 'localhost:xxxx/' GET request when running Swarm (#15926)

* swarm: began work on GetHandleFile method re: issue #155

* swarm: now able to serve landing page template

* swarm: added landing page template

* swarm: landing page has working input

* swarm: fixed CSS issue in template

* swarm: deleted extra lines

* swarm: deleted time header and made redirect a relative path

* swarm: removed code mistakenly left

whisper: filters no longer get removed after a while

* swarm: initial instrumentation with go-metrics

* swarm: initialise metrics collection and add ResettingTimer to HTTP requests

* swarm: update metrics flags names. remove redundant Timer.

* swarm: rename method for periodically updating gauges

* swarm: finalise metrics after feedback

* swarm/network: always init kad metrics containers

* swarm/network: off-by-one index in metrics containers

* swarm, metrics: resolved conflicts

Without this, deprecated bzzr requests just return an empty response.

Signed-off-by: Lewis Marshall <lewis@lmars.net>

whisper: light client mode introduced

* core: flush out trie cache more meaningfully on stop

* core: upgrade legacy tests to chain maker

Fix race condition in whisperv6/peer.go

* core, vm, common: define constantinople fork, start implementation of shift instructions

* vm: more testcases

* vm: add tests for intpool erroneous intpool handling

* core, vm, common: fix constantinople review concerns

* vm: add string<->op definitions for new opcodes

* go-metrics: fork library and introduce ResettingTimer and InfluxDB reporter.

* vendor: change nonsense/go-metrics to ethersphere/go-metrics

* go-metrics: add tests. move ResettingTimer logic from reporter to type.

* all, metrics: pull in metrics package in go-ethereum

* metrics/test: make sure metrics are enabled for tests

* metrics: apply gosimple rules

* metrics/exp, internal/debug: init expvar endpoint when starting pprof server

* internal/debug: tiny comment formatting fix

eth, les, light: filter on logs only, derive receipts on demand

swarm: repeated --ens-api CLI flag (#14386)

* p2p: remove unused code

* p2p: remove unused imports

RPC Server readRequest method sets the serverRequest error service
value as the rpcRequest.method and this change sets it to the right
service value.

core: yeah, funny file, drop it