fix keypoolrefill perfomance and adjusted callback logic

[k9ert]

When a wallet is created, the keypoolrefill-function get called for each address of probably 50 addresses wasting lots of time to write a csv-file dozens of times. This fixes this behaviour (in wallet.py).

It also adjusts the behaviour of the SPECTER_PERSISTENCE_CALLBACK (or SPECTER_PERSISTENCE_CALLBACK_ASYNC for a script which would run in a thread) to make it more flexible. This env-var takes an executable which will get called any time the persistence-layer is modifying a file.
Example:

• You have written a script called /usr/local/bin/my_rsync_script.sh
• You specify that script like export SPECTER_PERSISTENCE_CALLBACK=/usr/local/bin/my_rsync_script.sh
• In the past, that script simply get called
• Now if gets additional parameters:
  • the mode which is either write or delete
  • the filepath
• So an example-call would be /usr/local/bin/my_rsync_script.sh write /home/someuser/.specter/config.json
• You can also specify it in the SPECTER_PERSISTENCE_CALLBACK_ASYNC and it'll be called in a thread.
  Those changes are in persistence.py.

Also this PR contains an autodetection-mechanism for Liquid-nodes. So in parallel to BTC_RPC_USER, BTC_RPC_PASSWORD, BTC_RPC_HOST, BTC_RPC_PORT, ELM_RPC_PROTOCOL we now also have ELM_RPC_USER, ELM_RPC_PASSWORD, ELM_RPC_HOST, ELM_RPC_PORT ELM_RPC_PROTOCOL.
This was a bit tricky as a node was, up to now, indifferent about the type (until it queried the RPC-interface). Now we need to specify the type of the node upfront in order to make the autodetection work if someone uses both autodetection-mechanisms at the same time.
Also, the NodeManager created an initial Node if there wasn't one. So we needed to adjust that and we create a second Liquid-Node if there isn't one (AND an env-var is existing). See the node_type as the first argument (which will then differ in which Env-vars to pick up):

            if os.environ.get("ELM_RPC_USER"):
                self.add_node(
                    node_type="ELM",
                    name="Blockstream Liquid",
                    autodetect=True,
                    datadir=get_default_datadir(node_type="ELM"),
                    user="",
                    password="",
                    port=7041,
                    host="localhost",
                    protocol="http",
                    external_node=True,
                    default_alias=self.DEFAULT_ALIAS,
                )
            self.add_node(
                node_type="BTC",
                name="Bitcoin Core",
                autodetect=True,
                datadir=get_default_datadir(),
                user="",
                password="",
                port=8332,
                host="localhost",
                protocol="http",
                external_node=True,
                default_alias=self.DEFAULT_ALIAS,

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[moneymanolis]

LGTM!