Take nodes availability into account when choosing support nodes for aliens

[ikopylov]

Current implementation does not check nodes availability status and therefore leads to a decrease in efficiency and reliability.

https://github.com/qoollo/bob/blob/eab98a3cd1614b1eae1df20809559bbfb3f20573/bob/src/cluster/quorum.rs#L142 https://github.com/qoollo/bob/blob/ab2b97c32fed59fe31aad50c1610ab3891777a5c/bob-common/src/mapper.rs#L118