[addrman] Add Add_() inner function, fix Add() return semantics

Previously, Add() would return true if the function created a new
AddressInfo object, even if that object could not be successfully
entered into the new table and was deleted. That would happen if the new
table position was already taken and the existing entry could not be
removed.

Instead, return true if the new AddressInfo object is successfully
entered into the new table. This fixes a bug in the "Added %i addresses"
log, which would not always accurately log how many addresses had been
added.

p2p_addrv2_relay.py and p2p_addr_relay.py need to be updated since they
were incorrectly asserting on the buggy log (assuming that addresses are
added to addrman, when there could in fact be new table position
collisions that prevent some of those address records from being added).

src/addrman.h

@@ -76,7 +76,8 @@ public:
      * @param[in] source          The address of the node that sent us these addr records.
      * @param[in] nTimePenalty    A "time penalty" to apply to the address record's nTime. If a peer
      *                            sends us an address record with nTime=n, then we'll add it to our
-     *                            addrman with nTime=(n - nTimePenalty). */
+     *                            addrman with nTime=(n - nTimePenalty).
+     * @return    true if at least one address is successfully added. */
     bool Add(const std::vector<CAddress>& vAddr, const CNetAddr& source, int64_t nTimePenalty = 0);
 
     //! Mark an entry as accessible, possibly moving it from "new" to "tried".