change trough dedup date type to varchar

This is a backwards-compatible change whose purpose is to clarify the
existing usage.

In sqlite (and therefore trough), the datatypes of columns are just
suggestions. In fact the values can have any type. See
https://sqlite.org/datatype3.html. `datetime` isn't even a real sqlite
type.

Warcprox stores a string formatted like '2019-11-19T01:23:45Z' in that
field. When it pulls it out of the database and writes a revisit record,
it sticks the raw value in the `WARC-Date` header of that record.
Warcprox never parses the string value.

Since we use the raw textual value of the field, it makes sense to use a
textual datatype to store it.

warcprox/dedup.py

@@ -500,7 +500,7 @@ class TroughDedupDb(DedupDb, DedupableMixin):
     SCHEMA_SQL = ('create table dedup (\n'
                   '    digest_key varchar(100) primary key,\n'
                   '    url varchar(2100) not null,\n'
-                  '    date datetime not null,\n'
+                  '    date varchar(100) not null,\n'
                   '    id varchar(100));\n') # warc record id
     WRITE_SQL_TMPL = ('insert or ignore into dedup\n'
                       '(digest_key, url, date, id)\n'