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 ***
Add pthread_mutex_destroy in FakeLogger destructor
Merge pull request #160 from telecore-ivan/crash

Add mutex in FakeLogger to protect _lastlog string