Prepare to allow more complex expressions.

This enforces the "not nullptr" rule.

The duration of a song can have fractions of seconds
(quote from http://www.upnp.org/schemas/av/didl-lite-v2.xsd):

    The format of the duration string is:
    H+:MM:SS[.F+], or H+:MM:SS[.F0/F1]
    Where:
    +H              one or more digits to indicate elapsed hours,
    MM              exactly 2 digits to indicate minutes (00 to 59),
    SS              exactly 2 digits to indicate seconds (00 to 59),
    F+              any number of digits (including no digits) to indicate fractions of seconds,
    F0/F1   a fraction, with F0 and F1 at least one digit long,
                    and F0 < F1.
    The string may be preceded by an optional + or - sign, and the
    decimal point itself may be omitted if there are no fractional  seconds digits.

Until now, a duration with fractions of seconds could not be parsed and
resulted in an unknown duration. Only durations in the format "H+:MM:SS"
were feasible. This commit enables to read durations in the first format,
i.e. "H+:MM:SS[.F+]"

This is not necessary, because a strlen() on a literal gets optimized
away by the compiler.

We can do CURL requests asynchronously, and we don't need a
synchronous WorkQueue thread for that.

This allows parallelizing lookups and allows immediate cancellation.

Allows database plugins to use the EventThread, e.g. for CURL
integration.

We can remove those C++11 and C++14 kludges because we require GCC 4.9
now.

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

Implementations shall use exceptions instead.

For exception-safety.