Support uplink multi-slot allocations

Before this patch, allocate_usf() was implemented to only allocate 1 USF
per TBF, regardless of the available ul_slot mask.

As a result, only 1 slot at max was allocated to any TBF. That's a pity
because usual multislot classes like 12 support up to 2 UL slots per TBF
(in common TS with DL).

This patch reworks allocate_usf() to allocate as many UL multislots as
possible (given mslot class, current USF availability, TFI availability,
related DL TBF slots for the same MS, etc.).

As a result, it can be seen that AllocTest results change substantially
and maximum concurrent TBF allocation drops under some conditions.
That happens due to more USFs being reserved (because each TBF has now
more UL slots reserved). Hence now USF exhaustion becomes the usual
limitation factor as per the number of concurrent TBFs than can be
handled per TRX (as opposed to TFIs previously).

Some of the biggest limitations in test appear though because really
high end multislot classes are used, which can consume high volumes of
UL slots (USFs), and which are probably not the most extended devices in
the field.

Moreover, in general the curren timeslot allocator for a given
multislot class will in general try to optimize the DL side gathering
most of the possible timeslots there. That means, for instance on ms
class 12 (4 Tx, 4Rx, 5 Sum), 4 DL slots and 1 UL slot will still be
selected. But in the case where only 3 PDCHs are available, then with
this new multi-slot UL support a TBF will reserve 3 DL slots and 2 UL
slots, while before this patch it would only taken 1 UL slot instead of
2.

This USF exhaustion situation can be improved in the future by
parametrizing (VTY command?) the maximum amount of UL slots that a TBF
can reserve, making for instance a default value of 2, meaning usual
classes can gather up 2 UL timelosts at a time while forbidding high-end
hungry classes to gather up to 8 UL timeslots.

Another approach would be to dynamically limit the amount of allowed
reservable UL timeslots based on current USF reservation load.

Related: OS#2282
Change-Id: Id97cc6e3b769511b591b1694549e0dac55227c43

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