Use pools for GCM contexts

docs/config/security.en.md

@@ -25,7 +25,7 @@ Most of them can be set in /etc/vitastor/vitastor.conf and in etcd, but don't su
 - [vault_timeout_ms](#vault_timeout_ms)
 - [vault_error_timeout_sec](#vault_error_timeout_sec)
 - [vault_refresh_leeway_sec](#vault_refresh_leeway_sec)
-- [max_aes_xts_pool_size](#max_aes_xts_pool_size)
+- [max_cipher_pool_size](#max_cipher_pool_size)
 
 ## etcd_client_cert
 
@@ -141,10 +141,10 @@ Time (in seconds) to wait before retrying after receiving an error from Vault.
 Extra time (in seconds) before real Vault token lease_timeout to refresh it, just
 in case of system clock drift.
 
-## max_aes_xts_pool_size
+## max_cipher_pool_size
 
 - Type: integer
 - Default: 256
 
-Maximum number of OpenSSL encryption contexts cached in OSD memory. Probably
-doesn't require modification.
+Maximum number of OpenSSL cipher contexts cached in OSD memory, counted separately
+for each cipher and for encryption/decryption. Probably doesn't require modification.