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
Test deregistration of not registered clients.

This test reproduce a segmentation fault in flowtable.cpp
Check that the AoR is found before removing it
Merge pull request #163 from telecore-ivan/deregistration

De-registration of not registered client results in segmentation fault in bono
Merge branch 'master' of github.com:Metaswitch/sprout into dev