Improve perf of creating modified apphost

[steveharter]

When the modified apphost file is created for standalone apps, it first is a copy of the original apphost file and then the entire contents are replaced instead of just the specific section(s) necessary. This is inefficient; the file size is currently around 77K.

The code should be changed to use a stream approach and just replace the specific sections that actually change (only the hash values change).

This was prioritized due to changes in #1338.

[nguerrera]

We also read the whole file in to a managed byte array, which shouldn't be necessary.