This
a) makes the log db threaded by using QtConcurrent to run queries in a
different thread but still keeps ordering of the queries and always
only allows a single query at a time (QSql is not threadsafe). This fixes
removeDevice calls failing if we take more than $jsonprc_timeout to clean
a deleted device from the DB and keeps nymead responsive during that too.
b) generally improces performance of the system by not requiring operations
(emitting events, changing states) to wait for the sync log db entry to be
made.
c) drops some of the houskeeping code on nymea startup. While this will still
do log db housekeeping if the DB exceeds maxDbSize, it will not run
housekeeping on the DB any more at application startup. Reasoning for this
is that there have been reports of rules/log entries beimg destroyed if a
plugin can't be found at application startup. Given our general direction
of working towards more dynamic plugin loading, this might happen more often
in the future and we sure don't want to destroy rules etc when we just
temporarily miss a plugin.
d) tries to fix issue #226 by rotating the DB not only when it fails to open
initially, but also when it fails to insert new entries.
