[fix](FileReader) broker reader is not thread-safe and can't be prefetched (#19321)
Fix errors when using brokers to load csv/json files:
5# doris::ClientCacheHelper::reopen_client(std::function<doris::ThriftClientImpl* (doris::TNetworkAddress const&, void**)>&, void**, int) [clone .cold] at /root/doris/be/src/runtime/client_cache.cpp:84
6# doris::io::BrokerFileReader::read_at_impl(unsigned long, doris::Slice, unsigned long*, doris::io::IOContext const*) [clone .cold] at /root/doris/be/src/io/fs/broker_file_reader.cpp:104
7# doris::io::FileReader::read_at(unsigned long, doris::Slice, unsigned long*, doris::io::IOContext const*) at /root/doris/be/src/io/fs/file_reader.cpp:31
8# doris::io::PrefetchBuffer::prefetch_buffer() at /root/doris/be/src/io/fs/buffered_reader.cpp:71
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