mainloop: Reactivate the linked connection event with a non empty list

[dgoulet-tor]

Linked connections aren't woken up by libevent due to I/O but rather
artificially so we can, by chunks, empty the spooled object(s).

Commit 5719dfb (in 0.3.4.1-alpha) made it
that the schedule_active_linked_connections_event would be only called once at
startup but this is wrong because then we would never go through again the
active linked connections.

Fortunately, everytime a new linked connection is created, the event is
activated and thus we would go through the active list again. On a busy relay,
this issue is mitigated by that but on a slower relays or bridge, a connection
could get stuck for a while until a new directory information request would
show up.

Fixes #28717, #28912

[coveralls]

Pull Request Test Coverage Report for Build 3373

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.003%) to 59.86%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/or/main.c	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/or/hs_common.c	1	83.55%

Totals
Change from base Build 3263:	-0.003%
Covered Lines:	41783
Relevant Lines:	69801

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💛 - Coveralls