Consolidate ban functions (old version; just in case)

[dhruv]

Initially motivated by: https://github.com/bitcoin/bitcoin/pull/19219/files#r442410269

Issue

Minor change to setban: Prior to this PR, setban implementation behaves inconsistently. If the user bans 127.0.0.0/24, they are consequently unable to ban 127.0.0.0 but are able to ban 127.0.0.0/32.

i.e. the test shown here passes:

BOOST_AUTO_TEST_CASE(rpc_ban_inconsistent)
{
    BOOST_CHECK_NO_THROW(CallRPC(std::string("clearbanned")));
    BOOST_CHECK_NO_THROW(CallRPC(std::string("setban 127.0.0.0/24 add")));
    BOOST_CHECK_THROW(CallRPC(std::string("setban 127.0.0.0 add")), std::runtime_error);
    BOOST_CHECK_NO_THROW(CallRPC(std::string("setban 127.0.0.0/32 add")));
}

Root cause

• BanMan::IsBanned(CNetAddr& ip) returns true if ip is a member of any banned subnet in BanMan::m_banned
• BanMan::IsBanned(CSubNet& subnet) returns true if subnet is an exact match with any banned subnet in BanMan::m_banned (as opposed to a subset subnet which would be consistent with the overloaded implementation)
• setban implementation in src/rpc/net.cpp rejects the setban request if BanMan::IsBanned returns true

Fix

This PR has two objectives:

1. Make the aforementioned inconsistent behavior, consistent.
2. It's probably not technically correct to just reject any bans. There's no such thing as an "invalid" ban request (except for the obvious invalid, or malformed addresses, etc). This PR proposes the following:

Let (s0, t0) be an existing ban, where s0 is a subnet and t0 is the ban expiration time. The user is now trying to add another ban (s1, t1).

Case s1 ⊂ s0:
If t1 <= t0: do nothing, return success because the end goal is accomplished - this will minimize client error handling
If t1 > t0: insert (s1, t1) into BanMan

Case s1 ⊇ s0:
If t1 <= t0: insert (s1, t1) into BanMan
If t1 > t0: remove (s0, t0), add (s1, t1) into BanMan (this consolidation reduces overhead when checking if a peer is banned)

Case no overlap:
insert (s1, t1) into BanMan

Unban:
Unbanning will look for exact matches of ban entries and remove them.