Implemented CharacterSet-based SBuf::find_first_of and find_first_not_of
Moved CharacterSet as forward-declaration in SBuf.h
Moved CharacterSet's backing storage to std::vector<uint8_t> to gain speed at the expense of space.
Improved CharacterSet's constructor
Made CharacterSet's name parameter optional with default value.
Move CharacterSet::operator += to .cc file; change SBuf::find_first_not_of to match STL
Clarified documentatio for CharacterSet; moved CharacterSet::add and constructor to .cc file
Using std::copy_if for CharacterSet::operator +=
Merged from trunk
Improved CharacterSet documentation.
Fix missing include in rev.13186
Add unit tests for method parsing update
Merged from trunk
Reverted buggy change on CharacterSet::operator+=, documentation touches
Merge CharacterSet
Added send_hit and store_miss squid.conf directives
to control caching of responses using response info.

The existing "cache" directive is checked before Squid has access to the
response and, hence, could not use response-based ACLs such as http_status.
Response-based ACLs may be essential when fine-tuning caching. Squid Bug 3937
(StoreID can lead to 302 infinite loop) is a good use case.

Updated old "cache" directive documentation to provide more information, to
help folks distinguish the three related directives, and to polish for
clarity.

TODO: Support lookup_hit and possibly deprecate/remove "cache".
Rename SBuf::find_first_of and find_first_not_of to camelCase.
Fix missing include in rev.13190