Crash when making simultaneous gRPC calls on the same client

[ericgross2]

This happened on Linux RT, but the issue appears to be platform agnostic. It appears to be due to missing synchronization protecting the list of in-progress client calls.

[Current thread is 1 (Thread 0x7f507ae3d700 (LWP 6206))]
(gdb) bt
#0  raise (sig=11) at /usr/src/debug/glibc/2.24-r0/git/sysdeps/unix/sysv/linux/raise.c:58
#1  0x00007f50928f3cb1 in ?? () from /usr/local/natinst/labview/./liblvrt.so.21.0
#2  <signal handler called>
#3  0x00007f4fcafa4be3 in std::_List_iterator<grpc_labview::ClientCall*>::operator++ (this=0x7f507ae3cc80) at /usr/include/c++/7.3.0/bits/stl_list.h:162
#4  0x00007f4fcafa28f8 in std::__cxx11::list<grpc_labview::ClientCall*, std::allocator<grpc_labview::ClientCall*> >::remove (this=0x7f504c03ebc8, __value=@0x7f507ae3cd10: 0x7f5038000e10) at /usr/include/c++/7.3.0/bits/list.tcc:333
#5  0x00007f4fcaf9181c in CompleteClientUnaryCall2 (callId=0x7f5038000e10, responseCluster=0x13d7980 "\024", errorMessage=0x15f6398, errorDetailsCluster=0x15f6380) at /home/admin/grpc-labview/src/grpc_client.cc:302
#6  0x00000000015d0bf6 in ?? ()
#7  0x00000000015f5bc0 in ?? ()
#8  0x00007f5092bb5913 in ExtFuncWrapper () from /usr/local/natinst/labview/./liblvrt.so.21.0
#9  0x00000000015cf0f1 in ?? ()
#10 0x0000000000ff4bc0 in ?? ()
#11 0x00007f509340e460 in ?? () from /usr/local/natinst/labview/./liblvrt.so.21.0
#12 0x00007f509334c0c0 in ?? () from /usr/local/natinst/labview/./liblvrt.so.21.0
#13 0x0000000000000000 in ?? ()
(gdb)