The simple_transaction interface is slow. It requires 4 syscalls
(open, write, read, close) per query and shares a single lock for each
queries.

So replace its use with a compatible in multi_transaction interface.
It allows for a faster 2 syscall pattern per query. After an initial
open, an arbitrary number of writes and reads can be issued. Each
write will reset the query with new data that can be read. Reads do
not clear the data, and can be issued multiple times, and used with
seek, until a new write is performed which will reset the data
available and the seek position.

Note: this keeps the single lock design, if needed moving to a per
file lock will have to come later.
Signed-off-by: John Johansen <john.johansen@canonical.com>