shared/gatt-server: Add support for long write

With this patch long write and nested long write reliable is supported.
GATT server is responsible now to do aggregation of prep write data
for long write session.
Note: We consider long write as the consequtive prepare writes with
continues offsets.

E.g. 1

prep_write: handle 1, offset 0, value_len 10
prep_write: handle 1, offset 10, value_len 10
prep_write: handle 2, offset 0, value_len 10
prep_write: handle 2, offset 10, value_len 10

Will result with following calles to app:

exec_write: handle 1: offset 0, value_len 20
exec_write: handle 2: offset 0, value_len 20

E.g. 2

prep_write: handle 1, offset 0, value_len 10
prep_write: handle 1, offset 2, value_len 5
prep_write: handle 2, offset 0, value_len 10
prep_write: handle 2, offset 4, value_len 5

Will result with following calles to app:

exec_write: handle 1: offset 0, value_len 10
exec_write: handle 1: offset 2, value_len 5
exec_write: handle 2: offset 0, value_len 10
exec_write: handle 2: offset 4, value_len 5

E.g. 3
prep_write: handle 1, offset 0, value_len 10
prep_write: handle 1, offset 5, value_len 5
prep_write: handle 1, offset 10, value_len 6

will result with following calles to app:

exec_write: handle 1, offset 0, value 10
exec_write: handle 1, offset 5, value 11
1 file changed