Database: Support parenthesized JOINs

Database: Support parenthesized JOINs

SQL supports parentheses for grouping in the FROM clause.[1] This is
useful when you want to left-join against a join of other tables.

For example, say you have tables 'a', 'b', and 'c'. You want all rows
from 'a', along with rows from 'b' + 'c' only where both of those
exist.

 SELECT * FROM a LEFT JOIN b ON (a_b = b_id) JOIN c ON (b_c = c_id)

doesn't work, it'll only give you the rows where 'c' exists.

 SELECT * FROM a LEFT JOIN b ON (a_b = b_id) LEFT JOIN c ON (b_c = c_id)

doesn't work either, it'll give you rows from 'b' without a
corresponding row in 'c'. What you need to do is

 SELECT * FROM a LEFT JOIN (b JOIN c ON (b_c = c_id)) ON (a_b = b_id)

This patch implements this by extending the syntax for the $table
parameter to IDatabase::select(). When passing an array of tables, if a
value in the array is itself an array that is interpreted as a request
for a parenthesized join. To produce the example above, you'd do
something like

 $db->select(
     [ 'a', 'nest' => [ 'b', 'c' ] ],
     '*',
     [],
     __METHOD__,
     [],
     [
         'c' => [ 'JOIN', 'b_c = c_id ],
         'nest' => [ 'LEFT JOIN', 'a_b = b_id' ],
     ]
 );

[1]: In standards as far back as SQL-1992 (I couldn't find an earlier
 version), and it seems to be supported by at least MySQL 5.6, MariaDB
 10.1.28, PostgreSQL 9.3, PostgreSQL 10.0, Oracle 11g R2, SQLite 3.20.1,
 and MSSQL 2014 (from local testing and sqlfiddle.com).

Change-Id: I1e0a77381e06d885650a94f53847fb82f01c2694