Replace uses of boost::filesystem with fs

Step two in abstracting away boost::filesystem.

To repeat this, simply run:
```
git ls-files \*.cpp \*.h | xargs sed -i 's/boost::filesystem/fs/g'
```

src/addrdb.h

@@ -80,7 +80,7 @@ typedef std::map<CSubNet, CBanEntry> banmap_t;
 class CAddrDB
 {
 private:
-    boost::filesystem::path pathAddr;
+    fs::path pathAddr;
 public:
     CAddrDB();
     bool Write(const CAddrMan& addr);
@@ -92,7 +92,7 @@ public:
 class CBanDB
 {
 private:
-    boost::filesystem::path pathBanlist;
+    fs::path pathBanlist;
 public:
     CBanDB();
     bool Write(const banmap_t& banSet);