BUG/MEDIUM: server: set the macro for server's max weight SRV_UWGHT_MAX to SRV_UWGHT_RANGE

The max weight of server is 256 now, but SRV_UWGHT_MAX is still 255. As a result,
FWRR will not work well when server's weight is 256. The description is as below:

There are some macros related to server's weight in include/types/server.h:
    #define SRV_UWGHT_RANGE 256
    #define SRV_UWGHT_MAX   (SRV_UWGHT_RANGE - 1)
    #define SRV_EWGHT_MAX   (SRV_UWGHT_MAX   * BE_WEIGHT_SCALE)

Since weight of server can be reach to 256 and BE_WEIGHT_SCALE equals to 16,
the max eweight of server should be 256*16 = 4096, it will exceed SRV_EWGHT_MAX
which equals to SRV_UWGHT_MAX*BE_WEIGHT_SCALE = 255*16 = 4080. When a server
with weight 256 is insterted into FWRR tree during initialization, the key value
of this server should be SRV_EWGHT_MAX - s->eweight = 4080 - 4096 = -16 which
is closed to UINT_MAX in unsigned type, so the server with highest weight will
be not elected as the first server to process request.

In addition, it is a better choice to compare with SRV_UWGHT_MAX than a magic
number 256 while doing check for the weight. The max number of servers for
round-robin algorithm is also updated.

Signed-off-by: Godbach <nylzhaowei@gmail.com>