Processing msvcr120d.dll is very slow: 40s even with symcache

The first time, with no symcache, it takes 2:45 on this win10-1809 machine:
```
pre-module-load @0:00:06.546 in thread 7664
module load event: "MSVCR120D.dll" 0x00007ffab7520000-0x00007ffab7731000 modid: 7 C:\Windows\System32\msvcr120d.dll
<...>
post-module-load @0:02:51.521 in thread 7664
```

But with the symcache is still takes 40s!!
```
pre-module-load @0:00:04.531 in thread 4792
module load event: "MSVCR120D.dll" 0x00007ffab71e0000-0x00007ffab73f1000 modid: 7 C:\Windows\System32\msvcr120d.dll
<...>
post-module-load @0:00:45.966 in thread 4792
```

To repro run this test line:
```
$ bin64/drmemory -batch -- tests/unloadtest.exe tests/unloadlibMDd.dll 2
```