Auto-tuning work_mem and maintenance_work_mem

Josh Berkus suggested here that work_mem and maintenance_work_mem could
be auto-tuned like effective_cache_size:

    http://www.postgresql.org/message-id/50ECCF93.3060101@agliodbs.com

The attached patch implements this, closely matching the default values
for the default shared_buffers value:

    test=> SHOW shared_buffers;
     shared_buffers
    ----------------
     128MB
    (1 row)

    test=> SHOW work_mem;
     work_mem
    ----------
     1310kB
    (1 row)

    test=> SHOW maintenance_work_mem;
     maintenance_work_mem
    ----------------------
     20971kB
    (1 row)

Previous defaults were 1MB and 16MB, but the new defaults don't match
exactly because our max_connections is a power of 10 (100), not a power
of 2 (128).  Of course, if shared_buffer is 10x larger, those defaults
become 10x larger.

FYI, I based maintenance_work_mem's default on shared_buffers, not on
work_mem because it was too complex to change maintenance_work_mem when
someone changes work_mem.

I will work on auto-tuning temp_buffers next.  Any other suggestions?
wal_buffers is already auto-tuned.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +