Fix de-serialization bug where AddrMan is corrupted after exception

* CAddrDB modified so that when de-serialization code throws an exception Addrman is reset to a clean state
* CAddrDB modified to make unit tests possible
* Regression test created to ensure bug is fixed
* StartNode modifed to clear adrman if CAddrDB::Read returns an error code.

src/Makefile.test.include

@@ -60,6 +60,7 @@ BITCOIN_TESTS =\
   test/merkle_tests.cpp \
   test/miner_tests.cpp \
   test/multisig_tests.cpp \
+  test/net_tests.cpp \
   test/netbase_tests.cpp \
   test/pmt_tests.cpp \
   test/policyestimator_tests.cpp \