connmgr: Enforce max http seeder response size.
connmgr: Enforce max http seeder response size.

This imposes a limit on the response size from the http seeder such that
each returned entry is allowed to have max size of 256 bytes.  Since
there is also a limit of 16 entries per response, it amounts to a max
overall response size of 4KiB.

decred/dcrd,decred/dcrd