engine/ooapi: autogenerated API with login and caching (#234)

* internal/engine/ooapi: auto-generated API client

* feat: introduce the callers abstraction

* feat: implement API caching on disk

* feat: implement cloneWithToken when we require login

* feat: implement login

* fix: do not cache all APIs

* feat: start making space for more tests

* feat: implement caching policy

* feat: write tests for caching layer

* feat: add integration tests and fix some minor issues

* feat: write much more unit tests

* feat: add some more easy unit tests

* feat: add tests that use a local server

While there, make sure many fields we care about are OK.

* doc: write basic documentation

* fix: tweak sentence

* doc: improve ooapi documentation

* doc(ooapi): other documentation improvements

* fix(ooapi): remove caching for most APIs

We discussed this topic yesterday with @FedericoCeratto. The only
place where we want LRU caching is MeasurementMeta.

* feat(ooapi): improve handling of errors during login

This was also discussed yesterday with @FedericoCeratto

* fix(swaggerdiff_test.go): temporarily disable

Before I work on this, I need to tend onto other tasks.

* fix(ootest): add one more test case

We're going towards 100% coverage of this package, as it ought to be.

* feat(ooapi): test cases for when the probe clock is off

* fix(ooapi): change test to have 100% unittest coverage

* feat: sync server and client APIs definition

Companion PR: https://github.com/ooni/api/pull/218

* fix(ooapi): start testing again against API

* fix(ooapi): only generate each file once

* chore: set version to 3.7.0-alpha

While there, make sure we don't always skip a currently failing
riseupvpn test, and slightly clarify the readme.

* fix(kvstore): less scoped error message

internal/engine/ooapi/default_test.go

@@ -0,0 +1,41 @@
+package ooapi
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestDefaultTemplateExecutorParseError(t *testing.T) {
+	te := &defaultTemplateExecutor{}
+	out, err := te.Execute("{{ .Foo", nil)
+	if err == nil || !strings.HasSuffix(err.Error(), "unclosed action") {
+		t.Fatal("not the error we expected", err)
+	}
+	if out != "" {
+		t.Fatal("expected empty string")
+	}
+}
+
+func TestDefaultTemplateExecutorExecError(t *testing.T) {
+	te := &defaultTemplateExecutor{}
+	arg := make(chan interface{})
+	out, err := te.Execute("{{ .Foo }}", arg)
+	if err == nil || !strings.Contains(err.Error(), `can't evaluate field Foo`) {
+		t.Fatal("not the error we expected", err)
+	}
+	if out != "" {
+		t.Fatal("expected empty string")
+	}
+}
+
+func TestDefaultGobCodecEncodeError(t *testing.T) {
+	codec := &defaultGobCodec{}
+	arg := make(chan interface{})
+	data, err := codec.Encode(arg)
+	if err == nil || !strings.Contains(err.Error(), "can't handle type") {
+		t.Fatal("not the error we expected", err)
+	}
+	if data != nil {
+		t.Fatal("expected nil data")
+	}
+}