Update the default for zfs_txg_history

Update the default for zfs_txg_history

It's often useful to have access to txg history for debugging
purposes. This patch changes the default from 0 to 100 TXGs
worth of history preserved.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
Closes #6691