Receive request body content when content-length is present

[alcjzk]

PR contains a basic implementation for receiving request body content, as well as some fixes.

Main Changes

• Store headers in a map to protect against duplicates & better lookup.
• Basic parsing for Content-Length headers.
• Implemented a Buffer class to keep / separate & keep track of filled/unfilled portions of it.
• Implemented Reader::read_exact for reading an arbitrary amount of bytes
• Added basic reading of body content into ReceiveRequestTask
• Fixed #51

Not in this PR

• Validate Content-Length headers
• Transfer-encodings
• Buffer splitting sharing buffer between requests
• Limit the body size
• Growing buffers?

These will be continued on in #56 in favor of an earlier merge