THRIFT-5358: Add go.mod file to root directory

Client: go

This unblocks the development under go 1.16+, which starts to complain
when there's no go.mod file in any of the directories.

The current approach is certainly not the best solution ever, for
example it does not run the tests under lib/go/test/tests but copy them
into lib/go/test/gopath/src/sometest and run them there instead, but
those improvements can be done in the future in follow up PRs and this
should be a good enough first step to unblock developments.
diff --git a/build/docker/README.md b/build/docker/README.md
index 4ee57d1..44e30b0 100644
--- a/build/docker/README.md
+++ b/build/docker/README.md
@@ -178,7 +178,7 @@
 | dart      | 2.0.0         | 2.4.0         |       |
 | delphi    |               |               | Not in CI |
 | erlang    | 18.3          | 22.0          |       |
-| go        | 1.14.14       | 1.15.7        |       |
+| go        | 1.15.10       | 1.16.2        |       |
 | haxe      | 3.2.1         | 3.4.4         | THRIFT-4352: avoid 3.4.2 |
 | java      | 1.8.0_191     | 11.0.3        |       |
 | js        | Node.js 6.17.1, V8 5.1.281.111, npm 3.10.10 | Node.js 10.18.0, V8 6.8.275.32, npm 6.13.4 |     |
diff --git a/build/docker/ubuntu-bionic/Dockerfile b/build/docker/ubuntu-bionic/Dockerfile
index 699ae50..98f2607 100644
--- a/build/docker/ubuntu-bionic/Dockerfile
+++ b/build/docker/ubuntu-bionic/Dockerfile
@@ -140,9 +140,9 @@
       libglib2.0-dev
 
 # golang
-ENV GOLANG_VERSION 1.15.7
+ENV GOLANG_VERSION 1.16.2
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 0d142143794721bb63ce6c8a6180c4062bcf8ef4715e7d6d6609f3a8282629b3
+ENV GOLANG_DOWNLOAD_SHA256 542e936b19542e62679766194364f45141fde55169db2d8d01046555ca9eb4b8
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz && \
       echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - && \
             tar -C /usr/local -xzf golang.tar.gz && \
diff --git a/build/docker/ubuntu-disco/Dockerfile b/build/docker/ubuntu-disco/Dockerfile
index dd2df42..2c3166f 100644
--- a/build/docker/ubuntu-disco/Dockerfile
+++ b/build/docker/ubuntu-disco/Dockerfile
@@ -140,9 +140,9 @@
       libglib2.0-dev
 
 # golang
-ENV GOLANG_VERSION 1.15.7
+ENV GOLANG_VERSION 1.16.2
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 0d142143794721bb63ce6c8a6180c4062bcf8ef4715e7d6d6609f3a8282629b3
+ENV GOLANG_DOWNLOAD_SHA256 542e936b19542e62679766194364f45141fde55169db2d8d01046555ca9eb4b8
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz && \
       echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - && \
             tar -C /usr/local -xzf golang.tar.gz && \
diff --git a/build/docker/ubuntu-xenial/Dockerfile b/build/docker/ubuntu-xenial/Dockerfile
index bc66786..1184458 100644
--- a/build/docker/ubuntu-xenial/Dockerfile
+++ b/build/docker/ubuntu-xenial/Dockerfile
@@ -137,9 +137,9 @@
       libglib2.0-dev
 
 # golang
-ENV GOLANG_VERSION 1.14.14
+ENV GOLANG_VERSION 1.15.10
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 6f1354c9040d65d1622b451f43c324c1e5197aa9242d00c5a117d0e2625f3e0d
+ENV GOLANG_DOWNLOAD_SHA256 4aa1267517df32f2bf1cc3d55dfc27d0c6b2c2b0989449c96dd19273ccca051d
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz && \
       echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - && \
       tar -C /usr/local -xzf golang.tar.gz && \