Increase the mutations per commit limit to 80,000

[hengfengli]

Currently, this is set to 10,000 in

spanner-migration-tool/spanner/writer/batchwriter.go

Line 37 in 89802cf

countThreshold = 10 * 1000 // Spanner per-operation limit is 20,000.

. We should increase it to a higher limit as Spanner now supports 80,000 mutations per commit (doc).