Fix flaky data_iterator_test.go by adding locking

[grodowski]

Fixes fatal error: concurrent map read and map write that can occur when listeners are called concurrently

Example stacktrace:

=== RUN   TestDataIterator/TestDoneListenerGetsNotifiedWhenDone
fatal error: concurrent map read and map write

goroutine 187 [running]:
runtime.throw({0xb21635?, 0x0?})
	/opt/hostedtoolcache/go/1.18.10/x64/src/runtime/panic.go:992 +0x71 fp=0xc000a0ece8 sp=0xc000a0ecb8 pc=0x437b91
runtime.mapaccess1_faststr(0x465bfb?, 0xa417e0?, {0xc00029ccc0, 0xc})
	/opt/hostedtoolcache/go/1.18.10/x64/src/runtime/map_faststr.go:22 +0x3a5 fp=0xc000a0ed50 sp=0xc000a0ece8 pc=0x4148c5
github.com/Shopify/ghostferry/test/go.(*DataIteratorTestSuite).SetupTest.func1(0x10158a0?)
	/home/runner/work/ghostferry/ghostferry/test/go/data_iterator_test.go:64 +0x56 fp=0xc000a0edc0 sp=0xc000a0ed50 pc=0x92b636
github.com/Shopify/ghostferry.(*DataIterator).Run.func1.1(0xc0006f81e0)
	/home/runner/work/ghostferry/ghostferry/data_iterator.go:138 +0x304 fp=0xc000a0f1d8 sp=0xc000a0edc0 pc=0x881bc4
github.com/Shopify/ghostferry.(*Cursor).Each(0xc000a0f9f8, 0xc000a0f878)
	/home/runner/work/ghostferry/ghostferry/cursor.go:145 +0x3b7 fp=0xc000a0f6d0 sp=0xc000a0f1d8 pc=0x87d7f7
github.com/Shopify/ghostferry.(*DataIterator).Run.func1()
	/home/runner/work/ghostferry/ghostferry/data_iterator.go:[107](https://github.com/Shopify/ghostferry/actions/runs/12238239633/job/34135851396#step:5:108) +0x6d8 fp=0xc000a0ffe0 sp=0xc000a0f6d0 pc=0x880f38
runtime.goexit()
	/opt/hostedtoolcache/go/1.18.10/x64/src/runtime/asm_amd64.s:1571 +0x1 fp=0xc000a0ffe8 sp=0xc000a0ffe0 pc=0x46ab21
created by github.com/Shopify/ghostferry.(*DataIterator).Run
	/home/runner/work/ghostferry/ghostferry/data_iterator.go:71 +0x6ae
...

https://github.com/Shopify/ghostferry/actions/runs/12238239633/job/34135851396
https://github.com/Shopify/ghostferry/actions/runs/12239693187/job/34140746321