Handle application/x-pinos specially; track all the fd indices in the
buffers and send a release-fd message when the buffer is freed.
We can then use the payloader also in the client-source to handle the
release-fd messages correctly.
If we are using the original buffer fd without copying it into a temp
file we need to make sure the memory stays alive and is not reused until
all clients are done with it.
To do this, track what fd ids are in the outgoing buffers and ref the
original buffer. We also need to know when a message is sent to a client
and when that client does a release-fd on the fd index. This is done
with some new events on the multisocketsink.
Every time a message is sent to a client we hash the fd index in it and
ref the buffer in a per-client hash table (on the socket). Every time we
receive release-fd we remove the fd index from the hash and unref the
buffer again.
Clients that are killed get their socket removed, which also cleans up
the hashtable and frees the memory again.
Rename the payloader and depayloader to pinospay/pinosdepay because they
now store data in a pinos specific format.
Subclass GstElement because they are not real transform elements.
Rework/remove some buffer functions that are not used by some more
performant ones.
