[Filestore] directory handles don't survive filestore-vhost restart

[qkrorlqr]

Currently directory handles which are used in OpenDir, ReadDir, FsyncDir are stored in memory and don’t survive restart of filestore-vhost service.

Simple C program to reproduce the bug:

#include <stdio.h>
#include <dirent.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(int argc, char **argv) {
  int fd = open(argv[1], O_RDONLY);
  if (fd == -1) {
    perror("open");
    return 1;
  }
  printf("press enter: ");
	fflush(stdout);
  char buffer[1024];
  ssize_t bytes_read = read(STDIN_FILENO, buffer, sizeof(buffer));
  if (bytes_read == -1) {
    perror("read");
    return 1;
  }
  
  fsync(fd);
  DIR *dir = fdopendir(fd);
  if (dir == NULL) {
    perror("fdopendir");
    return 1;
  }
  struct dirent *entry;
  while ((entry = readdir(dir)) != NULL) {
    printf("%s\n", entry->d_name);
  }
  closedir(dir);
  return 0;
}