v3.5.15

Changes to squid-3.5.15 (23 Feb 2016):

- Bug 3870: assertion failed: String.cc: 'len_ + len <65536' in ESI::CustomParser
- Fix multiple assertion on String overflows
- Fix unit test errors on MacOS
- Better handling of huge response headers. Fewer incorrect "Bug #3279" messages.
- Log noise reduction for eCAP