kv/Rocksdbstore: change output format for short txc dump.

Plus refactors txc dumping  code to permit Unit Testing for it.

Here is the new output sample:
Short form:
 DF:D*3,DF:O,MF:A,PF:O*2,PF:P,...*2
Verbose form:
PutCF(prefix = O, key = 'key', val len = 5)
PutCF(prefix = P, key = 'key2', val len = 6)
PutCF(prefix = O, key = 'key3', val len = 6)
DeleteCF(prefix = O, key = 'A1')
DeleteCF(prefix = D, key = 'A1')
DeleteCF(prefix = D, key = 'A2')
DeleteCF(prefix = D, key = 'A3')
MergeCF(prefix = A, key = 'A5', val len = 6)
<...>*2

Signed-off-by: Igor Fedotov <igor.fedotov@croit.io>