Make sure that users' changes to the config files are preserved.

Fixes #4440.

* implement `reload` by sending the HUP signal

According to the 0.99 release info* synapse now uses the HUP signal to reload certificates:

> Synapse will now reload TLS certificates from disk upon SIGHUP. (#4495, #4524)

So the matrix-synapse.service unit file should include a reload directive.

Signed-off-by: Дамјан Георгиевски <gdamjan@gmail.com>

turns out that 0.34.1.1+1 comes before 0.34.1.1+bionic (etc).

The version may only contain "~ 0-9 A-Z a-z + - ." (sorting in that order).

Option 1: replace "+" with something that sorts after +. Options are "-" (but
dpkg-source complains about that) or "." (but that would mean we couldn't
distinguish packaging-only changes from real changes).

Option 2: stick with + and just find something that sorts after 'xenial'. The
only options there are "-", "." (same problems as before), "z", and "+".

Hence, ++1. Sorry.

Otherwise people can't upgrade from matrix-synapse without removing it first

...  to allow installation alongside our matrix-synapse transitional package.