Add grpc window size config and change the default value

[gengliqi]

Add GrpcInitialWindowSize and GrpcInitialConnWindowSize and change their default value from 1GiB to 128MiB.

The default value 1GiB may cause a TiDB OOM issue, see pingcap/tidb#49353.

The initial default value came from this PR pingcap/tidb#3377 where the author referred to pingcap/tidb#3377 (comment) as the evidence to justify the 1GiB default value. In this referenced document, the http2-window size is set to 1MiB and http2-connection-window is set to 1GiB. However, this document did not clearly describe how the test was done. I guess the key factor in getting faster results is changing http2-window from 64KiB to 1MiB, not changing http2-connection-window to 1GiB because at the top of the doc said the speed was just 4MiB/s if not using http2.

There is a lot of evidence that 1GiB is too large.

1. If these two values are not set, grpc-go will use the dynamic window size and the maximum of this dynamic window size is 16MiB. https://github.com/grpc/grpc-go/blob/dadbbfa2863a67ed640aac924b7b7fd18b50a429/internal/transport/bdp_estimator.go#L30
2. In cockroachdb master branch, the default values of GrpcInitialWindowSize and GrpcInitialConnWindowSize are 2MiB and 32MiB, respectively. The maximum of these two values is 64MiB. https://github.com/cockroachdb/cockroach/blob/master/pkg/rpc/settings.go#L69-L70. (rpc: permit gRPC stream window sizes up to 64 MB cockroachdb/cockroach#111255)

As a result, changing both default values to 128MiB will not likely cause performance issues but will likely reduce many TiDB OOM issues.