oxenstored: keep track of each transaction's operations

oxenstored: keep track of each transaction's operations

A list of (request, response) pairs from the operations performed within the
transaction will be useful to support transaction replay.

Since this consumes memory, the number of requests per transaction must not be
left unbounded. Hence a new quota for this is introduced. This quota, configured
via the configuration key 'quota-maxrequests', limits the size of transactions
initiated by domUs.

After the maximum number of requests has been exhausted, any further requests
will result in EQUOTA errors. The client may then choose to end the transaction;
a successful commit will result in the retention of only the prior requests.

Signed-off-by: Jonathan Davies <jonathan.davies@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Reviewed-by: Euan Harris <euan.harris@citrix.com>
Acked-by: David Scott <dave@recoil.org>