optimization of remote operations

[gsmolk]

1. In backup we can avoid one round-trip per file, that we spend on remote file opening via fio_open. (Done)
2. File list could be compiled by remote agent. (Done).
3. Support --backup-pg-log flag (Done)

[gsmolk]

Compiling filelist(1.5kk files) on agent yielded the following results:
master

=====================================================================================================================================
 Instance  Version  ID      Recovery Time           Mode  WAL Mode  TLI    Time    Data  WAL  Zratio  Start LSN   Stop LSN    Status 
=====================================================================================================================================
 node      11       QALDEW  2020-05-19 21:48:45+03  FULL  STREAM    1/0  3m:52s  4864MB    0    1.00  2/95000028  2/95000198  DONE 

issue_205:

===================================================================================================================================
 Instance  Version  ID      Recovery Time           Mode  WAL Mode  TLI  Time    Data  WAL  Zratio  Start LSN   Stop LSN    Status 
===================================================================================================================================
 node      11       QALT07  2020-05-20 03:22:21+03  FULL  STREAM    1/0   42s  4864MB    0    1.00  2/9F000028  2/9F000198  DONE

[gsmolk]

local backup:

===================================================================================================================================
 Instance  Version  ID      Recovery Time           Mode  WAL Mode  TLI  Time    Data  WAL  Zratio  Start LSN   Stop LSN    Status 
===================================================================================================================================
 node      11       QAMKFU  2020-05-20 13:14:35+03  FULL  STREAM    1/0   22s  4864MB    0    1.00  2/A7000028  2/A7000198  DONE

ssh:

===================================================================================================================================
 Instance  Version  ID      Recovery Time           Mode  WAL Mode  TLI  Time    Data  WAL  Zratio  Start LSN   Stop LSN    Status 
===================================================================================================================================
 node      11       QAMKH5  2020-05-20 13:15:43+03  FULL  STREAM    1/0   42s  4864MB    0    1.00  2/A9000028  2/A9000198  DONE

[gsmolk]

Merged.