BUG-8056: place an upper bound on number of transactions processed

BUG-8056: place an upper bound on number of transactions processed

When transactions complete their preCommit step immediately we end
up scheduling the next transaction immediately in the call stack,
hence if that completes immediately we end up eating away our stack
until we hit StackOverflowError.

Limit the number of transactions we process as a reaction to a single
message so that stack usage is under control. Should we hit this
limit, schedule a contiuation, which will deal with the rest of
the transactions.

Change-Id: Iad2812c823bd8e91ad45020ac50f6a8626654afb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>