Fixed bug in leveldb backend.

Read of nonexistent object was returning object of length 0 instead of error.
Merge pull request #17 from slon/master

Bugfix for leveldb backend
Do not mess with rpath - this allows to work both with installed and only built binaries
dnet_send_ack() should log into notice log level
Use correct ID in dnet_send_cmd_single() - if we have ID, it should be used, but not sph->src.id
Better id logs in srw: we dump ID of the current command and ID of the original SPH request
FIxed spurious ack generation - job without blocking flag should not sent any data - since
it is non-blocking request, none is waiting for the answer/ack.
proper srw command split in  dnet_ioclient

When starting cmd as following:
dnet_ioclient -r addr:port:family -g1:2:3... -c "application-name@event event-data lives here"

first space. i.e. one just after application-name@event is used as a delimiter
to split event name (first part) and event data (second part after the space)

Previously this first space was sent as a part of data,
but now we split string at this space and do not send it as part of data.
Previously data sent to client was: ' event-data lives here'
Now it is                           'event data lives here'
Added fixme comment about all groups iteration - we should use groups in session, not whole route table
Fixed failure path in pool growing route. Do not grow non-blocking IO pool at all - this is not needed anymore after srw processing stopped blocking, and started postponing ack sending.
Do not use app_watcher blocking helper - enqueue directly. It works, since we do not send ack for blocking requests until it is processed - ack (or data) will be sent in job's destructor
Removed app_watcher class
Removed dead code
Elliptics version update: 2.20.2.0
Do not allow to start the same engine again