feat: make block indexing configurable (#10608)

This adds a new folder-level configuration `FullBlockIndex`. It controls
whether we maintain the block index for a given folder -- currently
that's always true, now it becomes possible to turn off. The block index
is used for lookup of blocks across files and folders. Effectively, when
syncing a change, for each block, we check:

1. Is the block already present in the old version of the file? If so,
we can reuse (copy) it without network transfer. **This check is always
possible.**
2. Is the block already present in any other file in this folder or
other folders? If so we can copy it. **This check is only possible with
the full block index.**
3. We must transfer the block over the network.

Maintaining the full block index is costly in time, I/O and database
size. With this PR, maintaining the full block index becomes the default
for send-receive and receive-only folders only, with it disabled for
send-only and receive-encrypted folders. The block index is never useful
for encrypted folders, as blocks are encrypted separate for each file.
It is also not useful for send-only folders by themselves, though the
data in the send-only folder could be reused by other receive-type
folders if it were enabled.

For very large folders it may make sense to disable the full block index
regardless of folder type and just accept the resulting decrease in data
reuse.

Disabling or enabling the option in the GUI causes the index to be
destroyed or rebuilt accordingly.

https://github.com/syncthing/docs/pull/1005

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

gui/default/assets/lang/lang-en.json

@@ -50,6 +50,7 @@
     "Automatically create or share folders that this device advertises at the default path.": "Automatically create or share folders that this device advertises at the default path.",
     "Available debug logging facilities:": "Available debug logging facilities:",
     "Be careful!": "Be careful!",
+    "Block Indexing": "Block Indexing",
     "Body:": "Body:",
     "Bugs": "Bugs",
     "Cancel": "Cancel",
@@ -251,6 +252,7 @@
     "Log tailing paused. Scroll to the bottom to continue.": "Log tailing paused. Scroll to the bottom to continue.",
     "Login failed, see Syncthing logs for details.": "Login failed, see Syncthing logs for details.",
     "Logs": "Logs",
+    "Maintain an index of all blocks in the folder, enabling reuse of blocks from other files when syncing changes. Disable to reduce database size at the cost of not being able to reuse blocks across files.": "Maintain an index of all blocks in the folder, enabling reuse of blocks from other files when syncing changes. Disable to reduce database size at the cost of not being able to reuse blocks across files.",
     "Major Upgrade": "Major Upgrade",
     "Mass actions": "Mass actions",
     "Maximum Age": "Maximum Age",
@@ -394,6 +396,7 @@
     "Staggered": "Staggered",
     "Staggered File Versioning": "Staggered File Versioning",
     "Start Browser": "Start Browser",
+    "Starting": "Starting",
     "Statistics": "Statistics",
     "Stay logged in": "Stay logged in",
     "Stopped": "Stopped",