dbExistsTable: Specialize for oracle

[detule]

Hi:

This should improve dbWriteTable performance when writing to an Oracle connection. ( See: #540 )

Notes:

• Ideally would have liked to insert this query based approach for Oracle in dbListTables. However, for dbListTables("OdbcConnection", we advertise pattern based argument matching which is specific to the SQLTables/connection_sql_tables API; can bend the query to also do something similar but it's a bit more involved so punted for now.
• Outstanding are connection pane performance issues for Oracle which are also related to usage of connection_sql_tables. Have an idea how to mitigate at least some of them ( listing catalogs, schemas, etc ). Will address subsequently.
• Also in this PR, moved one of the specializations of dbExistsTable from Connection.R to Table.R, together with the remaining specializations.

[detule]

Just a note. Was trawling the ORACLE server side logs, and figured out that our calls to SQLTables gets translated to something that looks like:

SELECT * FROM
  ( SELECT null table_qualifier, o1.owner table_owner, o1.object_name table_name, decode(o1.owner,'SYS', decode(o1.object_type,'TABLE','SYSTEM TABLE','VIEW', 'SYSTEM VIEW', o1.object_type), 'SYSTEM', decode(o1.object_type,'TABLE','SYSTEM TABLE','VIEW', 'SYSTEM VIEW', o1.object_type), o1.object_type) table_type, null remarks
    FROM all_objects o1 WHERE o1.object_type in ('TABLE', 'VIEW')
    UNION
    SELECT null table_qualifier, s.owner table_owner, s.synonym_name table_name, 'SYNONYM' table_type, null remarks
    FROM all_objects o3, all_synonyms s
    WHERE o3.object_type in ('TABLE','VIEW') AND s.table_owner= o3.owner AND s.table_name = o3.object_name
    UNION
    SELECT null table_qualifier, s1.owner table_owner, s1.synonym_name table_name, 'SYNONYM' table_type, null remarks
    FROM all_synonyms s1 WHERE s1.db_link IS NOT NULL
  ) tables
WHERE 1=1 AND table_owner='SA' ORDER BY 4,2,3

This doesn't look all too dis-similar compared to the custom dbExistsTable implementation in this patch:

• Looks like it includes synonyms. Probably worth looking into including those if we can make this query performant. Which brings me to second point.
• Not sure why no subsetting is done within each of the UNION queries? Only using the table_owner filter after the union?

[detule]

Abandoning in favor of more comprehensive solution here