(paused, may cancel) Rename .gen -> gen for go 1.16 compatibility

[Groxx]

Hopefully resolves #3980 , when complete.
In a nutshell: go hasn't liked .gen since some time in 2018, but 1.16 was just released and now it's enforced.
Doing anything with 1.16 with modules enabled produces errors like:

go.uber.org/cadence/workflow imports
go.uber.org/cadence/.gen/go/shared: malformed import path "go.uber.org/cadence/.gen/go/shared": leading dot in path element

So this commit:

• changes the generated source dirs from ".gen" to "gen"
• changes licensegen to look at the new path
• moves the old proto files from .gen into gen
• find-and-replaces all github.com/uber/cadence/{.gen,gen}
• to test: clean and rebuild proto + thrift to make sure it's stable (it is)

To be complete, we'll need to make a new release of go.uber.org/cadence with similar changes, and import it here.
Unfortunately that PR is a fair bit more complex, so I'll wait for that to be decided before continuing here: cadence-workflow/cadence-go-client/pull/1057

[coveralls]

Coverage decreased (-0.03%) to 64.623% when pulling ec5c7b5 on sixteen into 59a98cf on master.

[Groxx]

no longer necessary, Go un-broke this.