This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

These methods are not exception-safe; std::lock_guard should better be
used.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Convenient method that behave differently than Read, and that will be used by
tag scanners.

This method will return in case of error, if the whole data is read or is EOF
is reached.

Reduce header dependencies.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Replace InputPlugin attributes.

Replaces the method Close().

Merge some duplicate code.