Merge 'core: drop mutex on contents' from Pere Diaz Bou

There is no need to have mutexes on buffers, we will introduce mutexes
if we want later on a file level to introduce serializability.
This pr also includes a fix in the cacheflush procedure where a wait of
inflight writes weren't waited.

Reviewed-by: Pekka Enberg <penberg@ki.fi>

Closes #384

sqlite3/src/lib.rs

@@ -885,7 +885,7 @@ fn sqlite3_errstr_impl(rc: i32) -> *const std::ffi::c_char {
         "datatype mismatch",                    // SQLITE_MISMATCH
         "bad parameter or other API misuse",    // SQLITE_MISUSE
         #[cfg(feature = "lfs")]
-        "",      // SQLITE_NOLFS
+        "",     // SQLITE_NOLFS
         #[cfg(not(feature = "lfs"))]
         "large file support is disabled", // SQLITE_NOLFS
         "authorization denied",                 // SQLITE_AUTH