jsonpb: Fix marshaling of Duration (golang#1221)

Negative nanosecond should not have negative sign after decimal point.
Add check for max and min seconds.

Fixes golang#1219.

all: upgrade to google.golang.org/protobuf@v1.23.0 (golang#1131)

all: upgrade to google.golang.org/protobuf@v1.22.0 (golang#1114)

all: upgrade to google.golang.org/protobuf@v1.21.0 (golang#1081)

go.mod: set go version to go1.9 (golang#1055)

proto: use fixed UnmarshalState API

Change-Id: I08abfaed1bafd0d650988883be31faefbd312331
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/222679
Reviewed-by: Damien Neil <dneil@google.com>

all: use google.golang.org/protobuf v1.20.0

Change-Id: Ib6c3a09e41548008897bacd54aa8fc6939ea8350
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221786
Reviewed-by: Herbie Ong <herbie@google.com>

all: use new protogen options API

Change-Id: Ib7ef4f9e8a6b7ada1f0cba4ae706be9fb8f7a10c
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221559
Reviewed-by: Damien Neil <dneil@google.com>

ptypes: regenerate with v3.11.4, update for go1.14 gofmt (golang#1045)

Update .proto files to github.com/protocolbuffers v3.11.4.

Add go1.13 and go1.14 to .travis.yml.

Avoid one-line funcs in protoc-gen-go output to prevent variable
post-gofmt output with go1.14.

proto: add MessageV1, MessageV2, and MessageReflect

The MessageV1 and MessageV2 functions convert to/from the v1 and v2
message interfaces.
The MessageReflect function provides a reflective view over message.
These functions do not have an "Of" suffix to be consistent with
the existing MessageName and MessageType functions.

Furthermore, we drop the "Of" suffix from functions in the descriptor
package to be consistent. This is a safe change since none of those
functions have seen a stable release.

We move the descriptor.GeneratedXXX types to the proto package
for documentation purposes.

Fixes golang#956

Change-Id: I566b74367798e2e3399db9902b58ffeb673199ca
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/219137
Reviewed-by: Damien Neil <dneil@google.com>