Add show tables with schema

[apstndb]

This PR adds named schemas support as SHOW TABLES <schema> command.
This is implemented as an extension of the existing SHOW TABLES to SHOW TABLES [<schema>], but it is explained as another command in README.md.

spanner> SHOW TABLES;
+----------------------------------------+
| Tables_in_apstndb-sampledb3            |
+----------------------------------------+
| Albums                                 |
| Concerts                               |
| Singers                                |
| Songs                                  |
+----------------------------------------+
71 rows in set (0.18 sec)

spanner> SHOW TABLES sch1;
+-----------------------------+
| Tables_in_apstndb-sampledb3 |
+-----------------------------+
| Albums                      |
| Singers                     |
| SingerView                  |
| Songs                       |
+-----------------------------+
4 rows in set (0.18 sec)

Related #172

[yfuruyama]

Left some minor comments, but LGTM. Thanks for submitting this change!

[yfuruyama]

Thanks for the additional change. LGTM!