build: upgrade to go 1.12

[nvanbenschoten]

NOTE: We are not planning on upgrading for the 19.1 release.

Known issues:

• (storage/engine: fix Go 1.12 build due to missing C.bool type casts #35195) cgo compilation error due to missing C.bool type casts.
• go vet -shadow option no longer supported. See the release notes for details on a fix.
• (util/log: use runtime.CallersFrames instead of runtime.Callers #17770) use runtime.CallersFrames instead of runtime.Callers.
• (sql: go 1.12 makes tests fail in sqlbase #35636, sql/sqlbase: fix TestEncDatumSize under go1.12 #35645) test failuers in sqlbase

[bdarnell]

Even though 19.2 will be the first CockroachDB release to officially build with Go 1.12, we should probably backport the changes here to our supported release branches (at least 2.1 and 19.1) for developer convenience (so that people who have upgraded to newer go versions don't have to maintain a separate toolchain or work entirely in docker).

[knz]

Folded into #35637.