The git branch feature/logger_db_watch is getting ready for merging into main midas.

The main change in the logger is the switch from db_open_record() to db_watch() as the 
method of listening to ODB variables. The new db_watch() function makes it cheap (in the 
number of hotlinks used) to implement "per-variable" history as the new default. In the new 
code, the old "per-equipment" history is no longer available.

In other words:

old per-equipment history: 1 hotlink per equipment
old per-variable history: 1 hotlink per "ls" entry in /eq/xxx/variables (big experiments can 
easily exceed the maximum number of hotlinks!)
new (per-variable) history: (back to) 1 hotlink per equipment

Notable changes from old history:

- works as described in my recent notes - the new code will complain about all incorrect use 
of history - where the old code sometimes silently malfunctioned (i.e. symlinks in unexpected 
places) or bombed (i.e. infinite loop reloading the history).
- all references to "PerVariableHistory" in ODB are removed (this is the new default)
- the "structured bank" records (subdirectories under variables, as in /eq/xxx/var/struct/value) 
are now broken up into individual items. This change is forced by the difference between 
db_open_record() and db_watch() for structured banks written using db_set_record(). The old 
per-variable history kept these items together in one event.

This change is also inline with Stefan's suggestion that all compound items, including arrays, 
should be broken up into separate history events. Keeping with this suggestion, right now 
only arrays are not broken up - because of limitations in the history storage level. As history 
storage is improved, arrays will also be broken up into individual elements.

The new code is functionally complete and all are welcome to try it (but beware as it may eat 
your odb or your history storage - make a backup!).

git checkout feature/logger_db_watch

K.O.