Resequencer: allow to immediately issue documents

[spoeschel]

So far the resequencer starts to regularly issue documents only after
the first EBU-TT Live document has been received. However for some use
cases this might be inconvenient as an active document must exist at all
times e.g. when creating segments for an MPEG-DASH stream. The reason
for documents being issued only after the first received EBU-TT Live
document is that certain parameters of that document are used for
initialisation.

This commit adds a new configuration parameter that specifies a document
which will be used for initialisation (instead of the first received
EBU-TT Live document). Therefore the resequencer will immediately (be
able to) start issuing documents after its creation.

Closes #505.

[nigelmegitt]

Hi @spoeschel please could you rebase this on the release/2.1.2 branch? Our release planning pattern here is to open pull requests against a release stage branch, then when we have a good candidate for release, merge that wholesale into master and add a release tag.

[spoeschel]

Yes, I rebased the MR branch and (force) pushed it again.

[nigelmegitt]

Thank you! @frans-ebu is on the case to sort out the travis build issue.

[nigelmegitt]

@spoeschel the travis build issue has now been fixed but I think it may need yet another rebase, apologies.

[spoeschel]

Closing, because of #503 (comment).