transaction example (and others) fails with "file already exists" on pool create

[igchor]

ISSUE: transaction example (and others) fails with "file already exists" on pool create

Environment Information

• libpmemobj-cpp version(s): 1.11
• PMDK (libpmemobj) package version(s):
• OS(es) version(s):
• kernel version(s):
• compiler, libraries, packaging and other related tools version(s):

Please provide a reproduction of the bug:

Run ./examples/example-transaction

How often bug is revealed:

always

Actual behavior:

Command fails with "Exception Failed creating pool: file poolfile already exists"

Expected behavior:

Command succeeds

Details

We should make sure, the poolfile is different for each tx example function. To do that I suggest making path to the pool a parameter for the functions. This would also allow us to run all examples as tests (with a proper cleanup).

Additional information about Priority and Help Requested:

Are you willing to submit a pull request with a proposed change? (Yes, No)

Requested priority: (Showstopper, High, Medium, Low)