Default buffer size is too small.

[methane]

Issue description

Current default buffer size is 4K. It is too small.
This buffer size is grow when receiving large packet. But when receiving huge numbers of rows, this size is not grown.

mysqlclient default buffer size is 16KiB.
https://dev.mysql.com/doc/refman/8.0/en/mysql-command-options.html#option_mysql_net-buffer-length

[methane]

I added a benchmark to query 10k rows in #1415.
I can not find significant performance difference between 4k and 16k buffer.
Maybe, it is because I used virtual network in macbook. (I used Rancher Desktop and Docker MySQL)
I need to run this benchmark with real network.

goos: darwin
goarch: arm64
pkg: github.com/go-sql-driver/mysql
                           │  buf4k.txt   │           buf16k.txt            │
                           │    sec/op    │    sec/op     vs base           │
ReceiveMassiveRows/query-8   5.698m ± ∞ ¹   5.719m ± ∞ ¹  ~ (p=1.000 n=1) ²
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05

                           │   buf4k.txt   │            buf16k.txt            │
                           │     B/op      │     B/op       vs base           │
ReceiveMassiveRows/query-8   859.8Ki ± ∞ ¹   859.8Ki ± ∞ ¹  ~ (p=1.000 n=1) ²
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05

                           │  buf4k.txt   │           buf16k.txt            │
                           │  allocs/op   │  allocs/op    vs base           │
ReceiveMassiveRows/query-8   50.00k ± ∞ ¹   50.00k ± ∞ ¹  ~ (p=1.000 n=1) ²
¹ need >= 6 samples for confidence interval at level 0.95
² all samples are equal