only make NewDefaultServer once when neccessary

[greedy52]

reapply #1 with sync.OnceValue

before:

stevehuang@mac ~/go/github.com/gravitational/teleport/lib/srv/db/mysql (master) $ GODEBUG='inittrace=1' go test  2>&1 | grep "go-mysql\/server"
init github.com/go-mysql-org/go-mysql/server @41 ms, 188 ms clock, 1528944 bytes, 13261 allocs

after:

stevehuang@mac ~/go/github.com/gravitational/teleport/lib/srv/db/mysql (master) $ GODEBUG='inittrace=1' go test  2>&1 | grep "go-mysql\/server"
# github.com/go-mysql-org/go-mysql/server
init github.com/go-mysql-org/go-mysql/server @35 ms, 0.019 ms clock, 792 bytes, 26 allocs

[rosstimothy]

Thanks Steve! Is there any reason that we are using teleport-v1.9.1 as the base branch for our changes in this repo? Our typical pattern for forks is to use master/main to track and pull in upstream changes, and to use teleport as the default branch for our custom tweaks.