#2557 - ESP8266WebServer uses >2kb of stack when reinitialized in setup()

[chschu]

This PR for #2557 changes the _currentUpload field in the ESP8266WebServer class to std::unique_ptr<HTTPUpload>, and allocates/deallocates it on demand.

Commit 67f4fc4 eliminates most of the exit paths in ESP8266WebServer::handleClient. It moves the client reset and yield() call to the end of the function, introduces else clauses to remove exit paths, and inverts if/else constructs to simplify the function.