Time to move on, two years after 2017.

The "end of stream" packet is not special; it contains normal data,
and thus we should pass it to OnOggPacket().

This fixes one part of https://github.com/MusicPlayerDaemon/MPD/issues/601

Fixes regression by commit 28e07e90

Closes https://github.com/MusicPlayerDaemon/MPD/issues/596

Really drain all data from the period_buffer in any case.

This assertion is not about whether all data has been consumed, but
about whether there has been any data at all.

There cannot be partial frames in the buffer, so we don't need
GetPeriodPosition(); it's enough to check whether head has been moved
in this period.

These options are exclusive.

bad_alloc depends on <new>

Eliminates a number of allocations, because callers don't need to copy
the strings to a newly allocated buffer only to null-terminate them.
And most callers don't need to have a null-terminated string.

Prepare for templated functions.

Prepare the transition to std::mutex.

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

A first use case for our libchromaprint integration added by commit
30e22b75

Let the constructor and destructor do this.  This means that all users
have to be converted to allocate PcmConvert dynamically.