Use separate event counters for bitmaps

Use independent event_counter_t for each bitmap. Each is 4k long so
as to make it independent of other bitmap events.

The idea is to keep each bitmap specific to the node and not change much of the bitmap code.

0                    4k                     8k                    12k
-------------------------------------------------------------------
| idle                | md super            | bitmap super        |
| ev counter[1]       | bm[1]               | bm[1, continued]    |
| ev_counter[2]       | bm[2]               | bm[2, continued]    |
| ev_counter[3]       | bm[3]               | bm[3, continued]    |

Questions:
1. Is updating events fields enough since it represents the dynamic fields of
   the bitmap superblock?
2. Not sure of the change to filemap_get_page()

TODO:
 * Fix for archs where PAGE_SIZE != 4k
 * Fix checks with respect to file_pages exceeding page->index
 * bad block logs if required
Communication Framework
Message types for different co-ordination

Includes send functions
Read a bitmap from other slots to gather resync data
Bitmap Lock for nodes bitmap