Add mutex in FakeLogger to protect _lastlog string

This fix a crash in the StatefulEdgeProxyTest.TestPreferredAssertedIdentities unit test

  [ RUN      ] StatefulEdgeProxyTest.TestPreferredAssertedIdentities
  *** glibc detected *** /home/ubuntu/sprout/build/bin/sprout_test: double free or corruption (!prev): 0x0000000001fa86a0 ***

  Core was generated by `/home/ubuntu/sprout/build/bin/sprout_test --gtest_output=xml:/home/ubuntu/sprou'.
  Program terminated with signal 6, Aborted.
  #0  0x00002abd3d14b425 in __GI_raise (sig=<optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
  64  ../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory.
  Traceback (most recent call last):
    File "/usr/lib/debug/usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.16-gdb.py", line 62, in <module>
      from libstdcxx.v6.printers import register_libstdcxx_printers
  ImportError: No module named libstdcxx.v6.printers
  (gdb) bt
  #0  0x00002abd3d14b425 in __GI_raise (sig=<optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
  #1  0x00002abd3d14eb8b in __GI_abort () at abort.c:91
  #2  0x00002abd3c9b969d in __gnu_cxx::__verbose_terminate_handler () at ../../../../src/libstdc++-v3/libsupc++/vterminate.cc:95
  #3  0x00002abd3c9b7846 in __cxxabiv1::__terminate (handler=<optimized out>) at ../../../../src/libstdc++-v3/libsupc++/eh_terminate.cc:40
  #4  0x00002abd3c9b7873 in std::terminate () at ../../../../src/libstdc++-v3/libsupc++/eh_terminate.cc:50
  #5  0x00002abd3c9b796e in __cxxabiv1::__cxa_throw (obj=0x2abd50009390, tinfo=<optimized out>, dest=<optimized out>)
      at ../../../../src/libstdc++-v3/libsupc++/eh_throw.cc:83
  #6  0x000000000058b799 in test_main_handle_signal (xiSigNum=11) at ut/test_main.cpp:124
  #7  <signal handler called>
  #8  __memcpy_ssse3_back () at ../sysdeps/x86_64/multiarch/memcpy-ssse3-back.S:1579
  #9  0x00002abd3c9a1842 in copy (__n=<optimized out>, __s2=0x4771f18 "hDR8nN+b/r\205",
      __s1=0x2abd50000a48 "Verbose pjsip: fake_tcplis:50 FAKE_TCP listener testnode:5060: got incoming FAKE_TCP connection from 1.2.3.4:49152\nVerbose pjsip: fake_tcps0x1f7 FAKE_TCP server transport created\nDebug pjsip: sip_endp"...)
      at /build/buildd/gcc-4.6-4.6.3/build/x86_64-linux-gnu/libstdc++-v3/include/bits/char_traits.h:273
  #10 _M_copy (__n=<optimized out>, __s=0x4771f18 "hDR8nN+b/r\205",
      __d=0x2abd50000a48 "Verbose pjsip: fake_tcplis:50 FAKE_TCP listener testnode:5060: got incoming FAKE_TCP connection from 1.2.3.4:49152\nVerbose pjsip: fake_tcps0x1f7 FAKE_TCP server transport created\nDebug pjsip: sip_endp"...)
      at /build/buildd/gcc-4.6-4.6.3/build/x86_64-linux-gnu/libstdc++-v3/include/bits/basic_string.h:354
  #11 std::string::_Rep::_M_clone (this=0x4771f00, __alloc=..., __res=<optimized out>)
      at /build/buildd/gcc-4.6-4.6.3/build/x86_64-linux-gnu/libstdc++-v3/include/bits/basic_string.tcc:633
  #12 0x00002abd3c9a18e0 in reserve (__res=<optimized out>, this=0x2058f70)
      at /build/buildd/gcc-4.6-4.6.3/build/x86_64-linux-gnu/libstdc++-v3/include/bits/basic_string.tcc:512
  #13 std::string::reserve (this=0x2058f70, __res=<optimized out>)
      at /build/buildd/gcc-4.6-4.6.3/build/x86_64-linux-gnu/libstdc++-v3/include/bits/basic_string.tcc:503
  #14 0x00002abd3c9a1e0d in std::string::append (this=0x2058f70, __str="Debug statistic.cpp:95: Send new value for statistic client_count, size 1\n")
      at /build/buildd/gcc-4.6-4.6.3/build/x86_64-linux-gnu/libstdc++-v3/include/bits/basic_string.tcc:334
  #15 0x0000000000597c73 in FakeLogger::write (this=0x2058f20,
      data=0x2abd3f5d1d10 "Debug statistic.cpp:95: Send new value for statistic client_count, size 1\n") at ut/fakelogger.cpp:78
  #16 0x0000000000556e2e in Log::write (level=5, module=0x801987 "\r\004", line_number=95, fmt=0x801a10 "\326\016") at log.cpp:113
  #17 0x000000000055afde in Statistic::reporter (this=0x1ec2b28) at statistic.cpp:95
  #18 0x000000000055b904 in Statistic::reporter_thread (p=0x1ec2b28) at statistic.cpp:144
  #19 0x00002abd3c6ece9a in start_thread (arg=0x2abd3f5d4700) at pthread_create.c:308
  #20 0x00002abd3d208ccd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
  #21 0x0000000000000000 in ?? ()