Control the maximum number of LMDB readers

[Kerollmops]

Feature name	How to enable	Description	Missing for stabilization
Index max readers	You can export the env variable to specify the maximum number of simultaneous read transactions. # Only the environment variable can set this parameter MEILI_EXPERIMENTAL_INDEX_MAX_READERS=100	Control the maximum number of LMDB read transactions at the same time	This parameter shouldn't be necessary and will be removed when we are sure a bug using a lot of read transactions is fixed.