TS-2316: header_rewrite: unify style

Submitted by: Alexey Ivanov <aivanov@linkedin.com>
Sponsored by: LinkedIn
TS-2316: header_rewrite: removed trailing spaces

Submitted by: Alexey Ivanov <aivanov@linkedin.com>
Sponsored by: LinkedIn
TS-2316: header_rewrite: fixed segfaults in ConditionPath::append_value()

Submitted by: Manjesh Nilange <mnilange@linkedin.com>
Sponsored by: LinkedIn
TS-2316: header_rewrite: added variable expansion support to add-header

Submitted by: Thomas Jackson <thjackso@linkedin.com>
Sponsored by: LinkedIn
TS-2316: header_rewrite: fix for crash in client header condition

Submitted by: Manjesh Nilange <mnilange@linkedin.com>
Sponsored by: LinkedIn