Context: add in memory model support. It is used by default when
JENA_BACKEND is not set or when some error happen during the specific
storage backend initialization
This patch also switches off transaction support for SDB storage. We always
get SDBException: already in transaction from begin method when the code
is invoked from several threads. The workaround is to disable explicit
Tx usage on SDB and leave it with auto-commit default support.