[FIX] hr_holidays: speedup Time Off dashboard

When opening the Time Off App, the JS performs an rpc to call
hr_leave_type.get_days_all_request. Inside this method there are
some __get__ on non-stored computed fields. This triggers
a recomputation of said fields in _compute_leaves. _compute_leaves
uses _get_employees_days_per_allocation.

When there is an hr.leave.type with request_unit = 'hour', you may
get lots of validated allocations by employee. When this is the case,
_get_employees_days_per_allocation gets quite slow, especially the last
part about 'Future available leaves'.

This commit optimizes this part of _get_employees_days_per_allocation.
In this part the allocations are grouped by employees
and Intervals instances are of the form (start, stop, records). This means
that for a given employee_id, all the records of one Intervals will have
the same start and stop values. This allows us to move the call to
_get_work_days_data_batch outside of the
for future_allocation_interval in future_allocation_intervals._items loop.
Because _get_work_days_data_batch is quite expansive, moving it out
grealty speeds up the _get_employees_days_per_allocation method.

Example speedup: in a database with 50 validated allocations for
the same hr.leave.type and employee_id, the timing of
_get_employees_days_per_allocation 7.39s -> 158ms

opw-3123457

closes odoo/odoo#116217

Signed-off-by: Kevin Baptiste <kba@odoo.com>