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/dbwrapper.cpp

@@ -90,7 +90,7 @@ static leveldb::Options GetOptions(size_t nCacheSize)
     return options;
 }
 
-CDBWrapper::CDBWrapper(const boost::filesystem::path& path, size_t nCacheSize, bool fMemory, bool fWipe, bool obfuscate)
+CDBWrapper::CDBWrapper(const fs::path& path, size_t nCacheSize, bool fMemory, bool fWipe, bool obfuscate)
 {
     penv = NULL;
     readoptions.verify_checksums = true;