[sync/server] wtc race maybe cause zero worker thread

# Description of problem

wtc race maybe cause zero worker thread here.
https://github.com/containerd/ttrpc-rust/blob/152ac12c30f65b79407d4411c40d8c86bc5ae949/src/sync/server.rs#L111

could reproduce it by add `sleep 1s` before
https://github.com/containerd/ttrpc-rust/blob/152ac12c30f65b79407d4411c40d8c86bc5ae949/src/sync/server.rs#L112

# Expected result

There is at least one worker thread for the accepted socket fd.

# Actual result
Mabye there is no worker thread for the accepted socket fd.

Start in src/sync/server.rs at the linked lines around 111-112 and reproduce the race by adding the reported 1-second sleep before line 112. Trace accepted socket handling and worker-thread creation, then verify that every accepted socket fd has at least one worker thread and that the reproduction no longer leaves it unserved.

Written by the indexing model from the issue text.