msgr: be a bit more verbose during shutdown

msgr: be a bit more verbose during shutdown

At least until this bug is found:

09.09.28 15:16:54.678277 140100507507024 -- 10.10.10.92:6800/5504/0 >> 10.10.10.88:6789/0/0 pipe(0x186ae90 sd=61 pgs=0 cs=0 ltx=1).accept replacing existing (lossy) channel
msg/SimpleMessenger.cc: In function 'void SimpleMessenger::reaper()':
msg/SimpleMessenger.cc:1925: FAILED assert(lock.is_locked())
 1: /usr/bin/cosd(_ZN15SimpleMessenger6reaperEv+0x59d) [0x45493d]
 2: /usr/bin/cosd(_ZN15SimpleMessenger4waitEv+0x4d) [0x45574d]
 3: /usr/bin/cosd(main+0xfe0) [0x44e770]
 4: /lib/libc.so.6(__libc_start_main+0xe6) [0x7f6befc145a6]
 5: /usr/bin/cosd(__gxx_personality_v0+0x1b9) [0x44d4e9]
 NOTE: a copy of the executable, or `objdump -rdS <executable>` is needed to interpret this.