hotfix: better CQI requests, especially for TDD

In TDD mode, CQI requests are not possible in special subframes (at
least for some TDD configurations, see 36.213 7.2.3 that says "a
DL subframe is valid if it does not contain a DwPTS field if the
length is less than  7680 Ts").

In the code, we simply disable CQI requests in special subframes,
no matter what the length of DwPTS.

A problem can arise if the DCI0 for a given UE are sent only in
those special subframes. In this case the UE will never report
CQI and the eNB will use low MCS for this UE, impacting performances.

Another, related, problem is when there are several UEs. There again
one UE might always get its DCI0 in special subframes and thus never
report CQI. There again, performance issues.

This commit is an attempt to improve the situation.

It does two things.

1 - tag the UE as schedulable in the function UE_is_to_be_scheduled
    if the cqi_req_timer is expired

2 - use cqi_req_timer as a criterium when ordering UEs for UL scheduling

The value chosen for the expiration of the cqi_req_timer in
UE_is_to_be_scheduled is quite high (300) because as the code is
today we may overschedule the UE for short bursts until we receive
a CQI from the UE. [TODO: fix the code properly to avoid this behavior.]

Note: the fairRR scheduler has not been analyzed and this commit may
      not fix anything in case the fairRR scheduler is used.