tpcc: close prepared statements before RefreshConn

[together-wang]

Summary

• close all TPCC prepared statements before refreshing a connection
• clear statement maps so the next run prepares statements only for the new connection
• reuse the same cleanup path during thread teardown
• add a regression test covering statement close and map cleanup

Fixes #208.

Why

When a TPCC connection is refreshed after a failed ping or at the configured refresh interval, the old prepared statements were left open while the new connection prepared another full statement set. Repeating this cycle can retain server-side prepared statements and their associated parser/plan-cache memory.

Closing the old statements before RefreshConn releases the old connection's statement resources before the new set is prepared.

Validation

• GOEXPERIMENT=jsonv2 go test ./tpcc ./pkg/workload -count=1
• GOEXPERIMENT=jsonv2 go test ./... -count=1
• GOEXPERIMENT=jsonv2 go vet ./...

[pingcap-cla-assistant]

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