freeze config before notifying handlers that server has started
freeze config before notifying handlers that server has started

Summary:
Call `thriftConfig_.freeze()` before notifying handlers that server started. This is to avoid a race condition that occurs sometimes where we attempt to update `StaticServerAttributes` in `ThriftServerConfig` after the server has started.

line 542 in the code below

https://www.internalfb.com/code/fbsource/[65499d0bd468]/fbcode/thrift/lib/cpp2/server/ThriftServerConfig.h?lines=537-544

Differential Revision: D38959340

fbshipit-source-id: d3eefd1a38e6890258eccdd449550a82c679f7ae

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