add support for vtexplain to override the dbname target

[demmer]

This change adds a --dbname option to the vtexplain binary so that
callers can simulate what vtgate will do when the application connects
with a specific database name.

Add a test to show how this can be used to bypass the v3 routing and
send queries directly to a shard.

As an example, I used the same queries as the "options" test in a new "target" test where the shard is forced to ks_sharded/-40:

--- data/test/vtexplain/multi-output/options-output.txt	2018-08-16 06:57:55.000000000 -0700
+++ data/test/vtexplain/multi-output/target-output.txt	2018-08-23 06:14:05.000000000 -0700
@@ -1,26 +1,18 @@
 ----------------------------------------------------------------------
 select * from user where email='null@void.com'

-1 ks_sharded/-40: select * from user where email = 'null@void.com' limit 10001
 1 ks_sharded/40-80: select * from user where email = 'null@void.com' limit 10001
-1 ks_sharded/80-c0: select * from user where email = 'null@void.com' limit 10001
-1 ks_sharded/c0-: select * from user where email = 'null@void.com' limit 10001

 ----------------------------------------------------------------------
 select * from user where id in (1,2,3,4,5,6,7,8)

-1 ks_sharded/-40: select * from user where id in (1, 2) limit 10001
-1 ks_sharded/40-80: select * from user where id in (3, 5) limit 10001
-1 ks_sharded/c0-: select * from user where id in (4, 6, 7, 8) limit 10001
+1 ks_sharded/40-80: select * from user where id in (1, 2, 3, 4, 5, 6, 7, 8) limit 10001

 ----------------------------------------------------------------------
 insert into user (id, name) values (2, 'bob')

-1 ks_sharded/c0-: begin
-1 ks_sharded/c0-: insert into name_user_map(name, user_id) values ('bob', 2) /* _stream name_user_map (name user_id ) ('Ym9i' 2 ); */ /* vtgate:: keyspace_id:da8a82595aa28154c17717955ffeed8b */
-2 ks_sharded/-40: begin
-2 ks_sharded/-40: insert into user(id, name) values (2, 'bob') /* _stream user (id ) (2 ); */ /* vtgate:: keyspace_id:06e7ea22ce92708f */
-3 ks_sharded/c0-: commit
-4 ks_sharded/-40: commit
+1 ks_sharded/40-80: begin
+1 ks_sharded/40-80: insert into user(id, name) values (2, 'bob')/* vtgate:: filtered_replication_unfriendly */
+2 ks_sharded/40-80: commit

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