proxywatchd: make checktime constants configurable

this requires only saving the last checked time in `tested`.
you can run the following sql statement to update the existing values
in the database:
sqlite3 proxylist.sqlite \
"update proxylist set tested=tested-(1800+(failed*3600)) where failed < 6"

config.ini.sample

@@ -9,6 +9,8 @@ timeout = 15
 submit_after = 200
 use_ssl = false
 debug = false
+checktime = 3600
+perfail_checktime = 3600
 
 [ppf]
 search = true