Fix potential onLoad failure from getCapabilities call

[ateucher]

See warning: https://www.r-project.org/nosvn/R.check/r-patched-solaris-x86/bcdata-00check.html

• Remove bcdc_get_wfs_records_xml from onLoad:
  • set get_capabilities_xml to NULL on pkg load
  • bcdc_get_wfs_records_xml() retrieves xml on first request and store for rest of the session
  • eliminate some redundant messages
• Wrap example in try(), add missing skip_if_net_down()

[boshek]

This is such an elegant solution.