[New feature] Allow specifying the hostnames in pool_hba

[New feature] Allow specifying the hostnames in pool_hba

The commit adds the support of hostnames to be used in the address field of
pool_hba records, previously only CIDR address was supported.

Along with allowing the hostnames in address field of the HBA record the commit
also made the following enhancements in the area.

-- pool_hba records are now completely parsed at the loading time and we now
   keep the structured data of records instead of raw record lines,  This
   saves the parsing at every new connection time and however little it may
   be but its a performance enhancement.

-- Enhanced parsing now gives the better descriptive error/log messages.

-- Better handling of auth-options field