Work around some hosts requiring session_save_handler = dbtable or piwik crashes

[mattab]

As reported inthe forum many times (one example) Using file based sessions on NFS mounted partitions can be very slow and result in Piwik not workig at all.

Often users stop using Piwik completely or miraculously find the FAQ explaining how to swtich to DB based sessions: http://piwik.org/faq/how-to-install/#faq_133

How could we reasonably work around these problems?

• detect each host and enable if we know host is incompatible?
• or can we detect that a particular config doesn't work with file based sessions?

See also: #2997

[mattab]

see #2997