The nodelist_idx for each node_t serves as a unique identifier for
the node, so we can use a bitarray to hold all the excluded
nodes, and then remove them from the smartlist.
Previously use used smartlist_subtract(sl, excluded), which is
O(len(sl)*len(excluded)).
We can use this function in other places too, but this is the one
that showed up on the profiles of 30291.
Closes ticket 30307.
This should please coverity, and fix CID 1415722. It didn't
understand that networkstatus_get_param() always returns a value
between its minimum and maximum values.
There are now separate modules for:
* the list of router descriptors
* the list of authorities and fallbacks
* managing authority certificates
* selecting random nodes
