Use unique_ptr< SecondaryEvent > to avoid memory leaks

[heplesser]

This PR fixes #3501 using the std::unique_ptr approach suggested by @med-ayssar in that issue.

@akorgor Could you test this with Valgrind? Could you also check against master that using unique_ptr does not slow down simulations?

@med-ayssar Could you review as well?

[med-ayssar]

Looks good.

[akorgor]

Comparing the Valgrind memory-leak logs of nest master to those of PR #3562, PR #3561, and
the combination of PRs #3561 and #3562 shows that these changes fix the memory leaks, leaving only SLI-related leaks.

[akorgor]

Measuring runtime and memory usage against both the unpatched NEST version and a temporary fix using delete &prototype after lines

nest-simulator/nestkernel/connection_manager.cpp

Lines 1688 to 1697 in b737f23

	while ( lcid < lcid_end )
	{
	std::vector< unsigned int >::iterator readpos = recv_buffer.begin() + positions_tid[ syn_id ][ lcid ];
	prototype << readpos;
	prototype.set_stamp( stamp );
	// send delivers event to all targets with the same source
	// and returns how many targets this event was delivered to
	lcid += connections_[ tid ][ syn_id ]->send( tid, lcid, cm, prototype );
	}

shows that runtime remains unchanged while memory usage improves.

[akorgor]

Thank you for this valuable fix!