The changes to mlogger implementing per-variable history have been committed to
svn. Revision 4145.

The rationale for these changes is roughly described in
https://ladd00.triumf.ca/elog/Midas/347

The main user-visible effect is reduction of data volume written to history
files and better integration with the history plot system in mhttpd.

The new functionality is disabled by default, pending review by Stefan (Except
for /history/tags stuff, which will be created by mlogger and used by mhttpd).
To enable it, set "/equipment/xxx/Common/PerVariableHistory" to 1 (type TID_INT).

In the "per-variable" mode, each entry in /equipment/xxx/variables is assigned
it's own event id and creates it's own events in the history file. In the
"classical" (or per-equipment) mode, all variables are assigned the same event
id (equal to the equipment id) and are written to disk at the same time.

In other words, in per-equipment mode, if there are 100 variables and 1 of them
is updated, all 100 numbers are written to disk. In per-variable mode, only the
one updated variable is written out.

The one point for review in this implementation is the assignment of event id's.
Committed code uses the formula "1000*eq_id + n" (i.e. variables in equipment id
2 get 2001, 2002, etc..., equipment id 3 get 3001, 3002, ...). This formula
works for most experiments, but as I understand is no good for some experiments
at PSI. Other than inventing a better formula that would work for everybody in
every case, one can also assign event id's manually by creating appropriate
entries in "/history/events".

This code has been used at CERN for running ALPHA since last Summer and it will
be used extensively at TRIUMF for T2K/ND280 slow controls. Per-variable history
is also required for the pending implementation of "history logged directly to
an SQL database", to be used at T2K/ND280.

If history (ahem) is any guide, we will now have a brief period of fixing merge
errors and "works for me" mistakes.

K.O.