Upgrade zap and fix url

[dragonly]

1. upgrade zap to 1.19.0
2. fix unified log format doc url

[tisonkun]

Thanks for your pull request @dragonly ! May I ask the reason you do the version bump?

[dragonly]

@tisonkun I am currently trying to optimize the general_log performance of tidb-server, and digged into some details of zap. I found some interesting enhancements in the following releases.

For example, in v1.18.0:

• zapcore: Add Buffered Writer uber-go/zap#961: Add zapcore.BufferedWriteSyncer, a new WriteSyncer that buffers messages in-memory and flushes them periodically.

And buffering is crucial for high performance logging like the general log feature.

I haven't done a thorough research, but I believe we can do something later to enhance the logging performance of TiDB itself, by leveraging the enhancements of new releases of zap!

[tisonkun]

Thanks for your explanation. LGTM.

Though we're using our own WriteSyncer IIRC, we cannot make use of zapcore.BufferedWriteSyncer for free.

[dragonly]

Though we're using our own WriteSyncer IIRC, we cannot make use of zapcore.BufferedWriteSyncer for free.

Actually we can wrap this in a zapcore.BufferedWriteSyncer, and everything seems just work.

And there are actually several different loggers in TiDB, and I can leverage zapcore.BufferedWriteSyncer in general_log first!