Two small performance improvements to header generation

These both apply to the proxy servers:

First, instead of consturction a dict just to use with `.update()` simply set a
set of keys in the headers dict.

Second, generating the headers for proxy requests once, no matter how many
backend servers are accessed (instead of once per backend server), they all get
the same headers.

Change-Id: Ib42bf7750e175a6ad9b777fa35655380feeafb93
Merge "Two small performance improvements to header generation"