Hang on Mu2eEventDisplay constructor

[brownd1978]

Mu2eEventDisplay can hang on construction, apparently due to a race condition between several threads. I've noticed this under various situations, rarely reproducible. A typical traceback in debug is:
#0 0x00007ffff768679a in __futex_abstimed_wait_common () from /lib64/libc.so.6
#1 0x00007ffff7688fa0 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libc.so.6
#2 0x00007ffff2b465d6 in mu2e::Mu2eEventDisplay::beginJob (this=0x2ba6f50) at
Mu2eEventDisplay/src/Mu2eEventDisplay_module.cc:284
0x00007ffff768679a in __futex_abstimed_wait_common () from /lib64/libc.so.6

The line is:
284 cv_.wait(lock);