res/res_odbc: Remove unneeded queries when determining if a table exists

This patch modifies the ast_odbc_find_table function such that it only performs
a lookup of the requested table if the table is not already known. Prior to
this patch, a queries would be executed against the database even if the table
was already known and cached.

Review: https://reviewboard.asterisk.org/r/4405/

ASTERISK-24742 #close
Reported by: ibercom
patches:
  patch.diff uploaded by ibercom (License 6599)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@431617 65c4cc65-6c06-0410-ace0-fbb531ad65f3