THRIFT-5897: Use Read Lock for Listener in TServerSocket.Accept()

Problem:
Accept() previously used two separate lock acquisitions:
RLock() to read interrupted field
Lock() (write lock) to read listener field
Write lock was unnecessary since listener is only being read, not modified
Write locks block all other goroutines (both readers and writers)

Solution:
Consolidate both field reads under a single RLock()/RUnlock() pair
Both interrupted and listener are read-only operations, so read lock is sufficient

Author: AtharvaChiplunkar12

lib/go/thrift/server_socket.go

@@ -69,15 +69,13 @@ func (p *TServerSocket) Listen() error {
 func (p *TServerSocket) Accept() (TTransport, error) {
 	p.mu.RLock()
 	interrupted := p.interrupted
+	listener := p.listener
 	p.mu.RUnlock()
 
 	if interrupted {
 		return nil, errTransportInterrupted
 	}
 
-	p.mu.Lock()
-	listener := p.listener
-	p.mu.Unlock()
 	if listener == nil {
 		return nil, NewTTransportException(NOT_OPEN, "No underlying server socket")
 	}