* Common.h -> Threading.h -> Errors.h -> Common.h
* Remove reduncdent #include "ByteBuffer.h" in headers
* Remove redundent #include "Auth/BigNumber.h" in headers
* Avoid multyply data copy at use some now dropped functions in BigNumber.
* Avoid copy fixed byte count from byte arrays with unknown real size created from BigNumber.
The SRP-6 specifications clearly say, that v and s are only to be
calculated on registering a user and changing his password; calculating
them on every login is plain waste.
