session: Purge epoch headers

Previously added `__NEOFS__NETMAP*` X-headers did not justify themselves:
 - no one has ever used them;
 - even if you try, it’s not clear how to work with them.

Data users operate with containers and objects, they are unaware and
uninterested in system time details. If an object is available, system
must be able to respond to them. Where and when exactly to look for data
is best known only to the storage system itself.

Closes #282.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>

proto-docs/session.md

@@ -285,18 +285,8 @@ Key name must be a unique valid UTF-8 string. Value can't be empty. Requests or
 Responses with duplicated header names or headers with empty values will be
 considered invalid.
 
-There are some "well-known" headers starting with `__NEOFS__` prefix that
-affect system behaviour:
-
-* __NEOFS__NETMAP_EPOCH \
-  Netmap epoch to use for object placement calculation. The `value` is string
-  encoded `uint64` in decimal presentation. If set to '0' or not set, the
-  current epoch only will be used.
-* __NEOFS__NETMAP_LOOKUP_DEPTH \
-  If object can't be found using current epoch's netmap, this header limits
-  how many past epochs the node can look up through. The `value` is string
-  encoded `uint64` in decimal presentation. If set to '0' or not set, only the
-  current epoch will be used.
+Headers with `__NEOFS__` prefix are reserved for system needs, regular clients
+should avoid using them.
 
 
 | Field | Type | Label | Description |

session/types.proto

@@ -137,18 +137,8 @@ message SessionToken {
 // Responses with duplicated header names or headers with empty values will be
 // considered invalid.
 //
-// There are some "well-known" headers starting with `__NEOFS__` prefix that
-// affect system behaviour:
-//
-// * __NEOFS__NETMAP_EPOCH \
-//   Netmap epoch to use for object placement calculation. The `value` is string
-//   encoded `uint64` in decimal presentation. If set to '0' or not set, the
-//   current epoch only will be used.
-// * __NEOFS__NETMAP_LOOKUP_DEPTH \
-//   If object can't be found using current epoch's netmap, this header limits
-//   how many past epochs the node can look up through. The `value` is string
-//   encoded `uint64` in decimal presentation. If set to '0' or not set, only the
-//   current epoch will be used.
+// Headers with `__NEOFS__` prefix are reserved for system needs, regular clients
+// should avoid using them.
 message XHeader {
   // Key of the X-Header
   string key = 1 [json_name = "key"];