Some API fixes/cleanup from PR #899 (#951)

Split out from Alex's PR for quicker inclusion into master.

* Supply `execution_context` deleter for `std::unique_ptr` via class, so
that a function does not have to be supplied as an argument at
construction.
* Supply `arb::basic_spike` by const reference to `ostream` writer.
* Add domain index to `connection` `ostream` writer.