added node finder features

[SimonSK]

Here are some features I'm adding to the client, assuming that we are building one tool as both node-finder and scanner. Some of them, like NoMaxPeers, may not be necessary, but I decided to include them anyways just in case we need them. (For example, it is possible that leaving the maximum number of peer connections set may limit the number of concurrent dial/accepts even if we increase the numbers).

• [x] allow setting maxActiveDialTasks (maximum number of concurrent dials)
• [x] allow setting maxAcceptConns (maximum number of concurrent incoming connections)
• [x] allow setting the maximum allowed number of file descriptors
• [x] allow unlimited number of peer connections (this option may not be necessary, but added anyways)
• [x] allow blacklisting ip address (in case someone requests not to be scanned)

[zzma]

Would it be possible to break this into multiple, smaller PRs? It's hard to review when there are tons of things going on. In general, we should try to PR smaller bits of functionality / code that don't break the build

[zzma]

But yes, it is a good idea to PR incrementally so we can provide feedback!

[SimonSK]

@zzma @sid1602 updated this PR (and the description) to include only the first few commits.