reduce TR_MULTISCRAPE_MAX

[reardon]

currently, TR_MULTISCRAPE_MAX = 64

this leads to scrape url's that are too long (~4KB) for many trackers, at least among the private trackers I surveyed. ie, trackers will return HTTP 414 when url is too long. Every tracker I tested was able to deal with 1036byte url, so recommend reducing this to TR_MULTISCRAPE_MAX = 15

(based on average hash length of 50bytes)

[cfpp2p]

This might have been a problem here too ticket:5300#comment:5
reference:

Up to about 74 torrents can be scraped at once.

​http://www.bittorrent.org/beps/bep_0015.html#udp-tracker-protocol

[cfpp2p]

@reardon thanks for the testing! I was able to patch so there's a setting I can use to change the value when those pesky HTTP 414 occur.