[addrman] inline Clear() into CAddrMan ctor

Clear() is now only called from the ctor, so just inline the code into
that function.

The LOCK(cs) can be removed, since there can be no data races in the ctor.

Also move the function definition out of the header and into the cpp file.

src/test/addrman_tests.cpp

@@ -48,7 +48,7 @@ public:
         unsigned char nVersion = 1;
         s << nVersion;
         s << ((unsigned char)32);
-        s << nKey;
+        s << uint256::ONE;
         s << 10; // nNew
         s << 10; // nTried