We import "github.com/pkg/errors" in the code, and actually put
the "github.com/pingcap/errors" there in the vendor path.
That's a dirty hack, and prevent the introduce of Go module.
- mytime: max/min configure with UTC tz, convert to local tz during process starting, convert it to session tz before checking bound.
- time: so `check()`/`add()` need force pass `StatementContext` parameter.
- builtin_time: pass `StatementContext` from `b.ctx.sessionVars` into check/add.
- dump: also set timezone for StatementContext with UTC.
- *_test/mock: fix test case, make mock context with stmt TimeZone.
- add testcase for checkTimestamp and mytime convert timezone.
