Certificate Validator buffer-overflow crashes Squid

Certificate Validator buffer-overflow crashes Squid

Certain server certificates with a large chain and/or large certificates
(i.e. due to excessive amount of SAN entries) are producing helper requests and
replies which are larger than the 32KB limit set in src/helper.cc.

The major problem for squid is that the helper response should fit in the
helper read buffer. Currently squid starts with 4K request buffer and if
required may increase the buffer size up to 32K.

This patch:
  - Uses a constant-size read buffer for helpers and accumulates the helper
    response to Helper::Reply object.
  - Changes the HelperServerBase::requests list to hold list of the new
    Helper::Xaction class which holds pairs of Helper::Request and
    Helper::Reply objects
  - Modifies the Helper::Reply class to accumulate data and restricts the
    required memory allocations

This is a Measurement Factory project