Merge from vendor branch TNF:

Merge from vendor branch TNF:
Version 9.1 delivers several features which users have been requesting for
years, removing roadblocks to deploying new or ported applications on
PostgreSQL. These include:
* Synchronous Replication: enable high-availability with consistency across
  multiple servers
* Per-Column Collations: support linguistically-correct sorting per database,
  table or column.
* Unlogged Tables: greatly improves performance for ephemeral data

Our community of contributors innovates with cutting-edge features. Version 9.1
includes several which are new to the database industry, such as:
* K-Nearest-Neighbor Indexing: index on "distance" for faster location and text
  search queries
* Serializable Snapshot Isolation: keeps concurrent transactions consistent
  without blocking, using "true serializability"
* Writeable Common Table Expressions: execute complex multi-stage data updates
  in a single query
* Security-Enhanced Postgres: deploy military-grade security and Mandatory
  Access Control