Add completion priority for each server or for vim-lsp globally

[E1k3]

Since vim-lsp is registered as asyncomplete source implicitly (i.e. without calling asyncomplete#register_source), I see no way to specify the priority of vim-lsp completion suggestions.
Because of this, the suggestions of other sources (e.g. asyncomplete-buffer) often appear above vim-lsp suggestions.
I do not need the priority to be an attribute of each individual lsp server, but IMO that would be one acceptable way to implement it.

[stale]

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