Feature/use signal cli json rpc

[rotdrop]

signal-cli comes with its own HTTP endpoint for JSON RPC calls since signal-cli 0.11.5.

This means that there is no longer a need for any intermediate wrappe, so lets just use the native HTTP endpoint of Signal-Cli.

The old methods are tried in turn if the native end-point is not reachable.

This PR maybe also fixes #573.

[rotdrop]

I just do not know what is the cause of the failure the that obscure PHPUnit pgsql / phpunit-pgsql-summary.

Also: the output of that funny test does not provide any information about how to fix this (probably: pseudo) failure.