Fast sticky session scheduler is broken

[vankoven]

As sticky session scheduler was introduced (#593) it was claimed to be the fastest scheduler. When a new request arrived destination server was chosen by HTTP scheduler call and subsequent group scheduler call. HTTP scheduler used a lot of string compare functions and can dramatically slow down system in environments with super long HTTP rules lists. Sticky sessions scheduler was intended to replace that expensive string operations with cheap sticky cookie calculations.

But recent #1017 patch broke this convention. VHost is assigned to a request, before sticky cookie is analysed. 'VHost assignment' operation is very close to old http scheduler operation: multiple string comparison operations may be involved. This means that Fast sticky sessions scheduler doesn't work any more.

'VHost assignment' operation is very important, since many configuration options have or will have vhost scope. To return Fast sticky scheduler back operational, target vhost must be saved into sticky cookie structure (TfwStickyConn).

[vankoven]

Should be fixed after #899: It moves obtaining of http sessions to earlier request processing stages.