Same payload are persisted multiple times

[qtcwt]

BEGIN_OP(payload) in a branch with retries (usually in nonblocking structures) would cause the same payload to be persisted for more than once, i.e., to be added into to_be_persisted list several times. This redundancy would affect the performance and increase the average size of to_be_persisted list.

One way to avoid that is to adjust existing API so that users can only set the birth epoch of payload during BEGIN_OP and add it to to_be_persisted list only when the linearization point has been passed (has succeeded). An example can be found in this thread in PR #4.

[qtcwt]

Well we may have to add payload to to-be-persisted bucket multiple times, since reset epoch in payload needs to be explicitly persisted after ABORT_OP in order to overwrite potential automatic writes-back after tagging but before resetting epoch. (relevant to Issue #7)

[qtcwt]

• revert experimental code in this thread