ES_EVENT_TYPE_NOTIFY_UNMOUNT: flush the cache off the ES handler thread

[tburgin]

We have seen some instances of com.google.santa.daemon being killed for not responding to ES AUTH messages:

Termination Reason:    Namespace ENDPOINTSECURITY, Code 2 

Thread 2::  Dispatch queue: BBReaderQueue
0   libsystem_kernel.dylib        	       0x189dc88d0 mach_msg_trap + 8
1   libsystem_kernel.dylib        	       0x189dc8d40 mach_msg + 76
2   libxpc.dylib                  	       0x189b57cc4 _xpc_pipe_mach_msg + 56
3   libxpc.dylib                  	       0x189b57554 _xpc_pipe_routine + 356
4   libEndpointSecurity.dylib     	       0x19ac8c404 _es_start_service + 76
5   libEndpointSecurity.dylib     	       0x19ac8c5e8 _es_clear_cache + 44
6   com.google.santa.daemon       	       0x104d601b0 -[SNTCachingEndpointSecurityManager flushCacheNonRootOnly:] + 184
7   com.google.santa.daemon       	       0x104d70ea4 __45-[SNTEndpointSecurityManager establishClient]_block_invoke + 1076
8   libEndpointSecurity.dylib     	       0x19ac90484 BBReader<ESMessageReaderConfig>::handleItems() + 272
9   libEndpointSecurity.dylib     	       0x19ac90260 BBReader<ESMessageReaderConfig>::woke(void*) + 28
10  libdispatch.dylib             	       0x189c421c8 _dispatch_client_callout + 20
11  libdispatch.dylib             	       0x189c45670 _dispatch_continuation_pop + 500
12  libdispatch.dylib             	       0x189c588e0 _dispatch_source_invoke + 1596
13  libdispatch.dylib             	       0x189c49784 _dispatch_lane_serial_drain + 376
14  libdispatch.dylib             	       0x189c4a438 _dispatch_lane_invoke + 444
15  libdispatch.dylib             	       0x189c4b704 _dispatch_workloop_invoke + 1784
16  libdispatch.dylib             	       0x189c54c98 _dispatch_workloop_worker_thread + 648
17  libsystem_pthread.dylib       	       0x189e02360 _pthread_wqthread + 288
18  libsystem_pthread.dylib       	       0x189e01080 start_wqthread + 8

It seems we are deadlocking ourselves if es_clear_cache() needs to spin anything up.

Also clang-format'ed the file.