Turn room query inside out.

The room query was very inefficient because filtered and summed all
room_date ranges before filtering by hotel. Instead we get the filtered
list of hotels and perform the sum only on ranges of eligible hotels.
Adding benchmark data.

Collected benchmark data on both hotel and room queries. Speedup for
room query is approximately 8x after adding 2 clustered indexes
HOTEL.(COUNTRY+CITY), ROOM_DATE.(SINGLE_ROOM_DATE) and query refactor.

Note that the system configuration for these tests is

  MSI x370 Notebook
  AMD E-350 Processor 1.60GHz
  4.00GB DDR3 RAM
  Seagate Momentus ST9500420AS 500GB 7200RPM SATA Disk

  Ubuntu Server 12.04 LTS 64-bit Running on VirtualBox 4.2.18
    8.00 GB Fixed Virtual Disk
    1296MB Physical RAM
    1 Physical CPU w/100% Physical Exectuion Cap
    Local NAT-based Networking
    IBM DB2/LINUXX8664 10.5.1 Database Server

The hardware is considered very modest for contemporary standards.