staircase: make it simpler

We get the base dates from the current series value dates.
For each of those we query one point, then concatenate
the result.

Conceptually much simpler to grasp.
Bonus side effect: staircase discrepencies of formula vs alias
extensions have disappeared.

Performance implications:

base:
DELTA all value dates 0.9304628372192383
DELTA 1 day  0.10948061943054199

correctness:
DELTA all value dates 1.174530267715454
DELTA 1 day  0.05773115158081055

So on the whole horizon we are a tad slower but there is an
interesting speedup already in a well-bounded case. This is not bad.

tshistory/tsio.py

@@ -286,26 +286,32 @@ class timeseries(SeriesServices):
         `delta` time after the insertion dates and where we
         keep the most recent ones
         """
-        histo = self.history(
-            cn, seriename, deltabefore=-delta,
+        if not self.exists(cn, seriename):
+            return
+
+        base = self.get(
+            cn, seriename,
             from_value_date=from_value_date,
-            to_value_date=to_value_date,
-            _keep_nans=True
+            to_value_date=to_value_date
         )
-        if histo is None:
-            return None
-
-        vimap = {}
-        vvmap = {}
-        for idate, series in histo.items():
-            for vdate, value in series.iteritems():
-                if vdate not in vimap or vimap[vdate] < idate:
-                    vimap[vdate] = idate
-                    vvmap[vdate] = value
+        if not len(base):
+            return base
+
+        chunks = []
+        for vdate in base.index:
+            ts = self.get(
+                cn, seriename, revision_date=vdate - delta,
+                from_value_date=vdate,
+                to_value_date=vdate
+            )
+            if ts is not None and len(ts):
+                chunks.append(ts)
 
-        ts = pd.Series(vvmap).sort_index().loc[from_value_date:to_value_date]
+        ts = pd.Series()
+        if chunks:
+            ts = pd.concat(chunks)
         ts.name = seriename
-        return ts.dropna()
+        return ts
 
     def exists(self, cn, seriename):
         return self._serie_to_tablename(cn, seriename) is not None